Court Opinion

ID: 9352615
Source: CourtListenerOpinion
Date Created: 2023-01-09 00:00:40.711693+00
Date Added: 2024-06-11T16:58:01.369212
License: Public Domain

Case: 20-51016     Document: 00516601811         Page: 1    Date Filed: 01/06/2023

           United States Court of Appeals
                for the Fifth Circuit                              United States Court of Appeals
                                                                            Fifth Circuit

                                                                          FILED
                                                                    January 6, 2023
                                  No. 20-51016                       Lyle W. Cayce
                                                                          Clerk

   Michael Cargill,

                                                           Plaintiff—Appellant,

                                      versus

   Merrick Garland, in his official capacity as U.S. Attorney General;
   United States Department of Justice; Steven
   Dettelbach, in his official capacity as Director of the Bureau of Alcohol,
   Tobacco, Firearms, and Explosives; Bureau of Alcohol, Tobacco,
   Firearms, and Explosives,

                                                        Defendants—Appellees.

                  Appeal from the United States District Court
                       for the Western District of Texas
                            USDC No. 1:19-CV-349

   Before Richman, Chief Judge, and Jones, Smith, Stewart,
   Dennis, Elrod, Southwick, Haynes, Graves, Higginson,
   Willett, Ho, Duncan, Engelhardt, Oldham, and Wilson,
   Circuit Judges.
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   Jennifer Walker Elrod, Circuit Judge, joined by Richman, Chief
   Judge, and Jones, Smith, Stewart, Southwick, Haynes,
   Willett, Ho, Duncan, Engelhardt, Oldham, and Wilson,
   Circuit Judges: ∗
           Since the National Firearms Act of 1934, federal law has heavily regu-
   lated machineguns. Indeed, as proposed, that law was known to many as “the
   Anti-Machine Gun Bill.” The possession or transfer of a machinegun was
   eventually banned through the Gun Control Act of 1968 and the Firearms
   Owners’ Protection Act of 1986. Today, possession of a machinegun is a fed-
   eral crime, carrying a penalty of up to ten years’ incarceration.
           This appeal concerns a regulation promulgated by the federal Bureau
   of Alcohol, Tobacco, Firearms, and Explosives, purporting to interpret the
   federal prohibition on machineguns as extending to bump stocks. A bump
   stock is a firearm attachment that allows a shooter to harness the natural re-
   coil of a semi-automatic weapon to quickly re-engage the trigger after firing,
   enabling him to shoot at an increased rate of speed. When ATF first consid-
   ered the type of bump stocks at issue here, it understood that they were not
   machineguns. ATF maintained this position for over a decade, issuing many
   interpretation letters to that effect to members of the public.
           But ATF reversed its longstanding position in 2018, subjecting anyone
   who possessed a bump stock to criminal liability. ATF reversed its position
   to a great extent in response to the tragic events that occurred in Las Vegas

           ∗
             Of the sixteen members of our court, thirteen of us agree that an act of Congress
   is required to prohibit bump stocks, and that we therefore must reverse. Twelve members
   (Chief Judge Richman and Judges Jones, Smith, Stewart, Elrod,
   Southwick, Haynes, Willett, Ho, Duncan, Engelhardt, and Wilson)
   reverse on lenity grounds. Eight members (Judges Jones, Smith, Elrod, Wil-
   lett, Duncan, Engelhardt, Oldham, and Wilson) reverse on the ground that
   federal law unambiguously fails to cover non-mechanical bump stocks.
            Chief Judge Richman, Judge Stewart, and Judge Southwick con-
   cur in the judgment and join in Part V, as does Judge Ho, who also writes separately.
   Judge Oldham concurs in the judgment and joins in Parts I–IV.A. Judge Haynes
   only concurs in the judgment and writes separately.

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   on October 1, 2017. On that day, a deranged gunman murdered dozens of
   innocent men and women, and injured hundreds more. To carry out this ap-
   palling crime, the gunman used many weapons and utilized many accesso-
   ries—including bump stocks.
          Public pressure to ban bump stocks was tremendous. Multiple bills to
   that effect were introduced in both houses of Congress. But before they could
   be considered in earnest, ATF published the regulation at issue here, short-
   circuiting the legislative process. Appellant Michael Cargill surrendered sev-
   eral bump stocks to the Government following publication of the regulation
   at issue. He now challenges the legality of that regulation, arguing that a
   bump stock does not fall within the definition of “machinegun” as set forth
   in federal law, and thus that ATF lacked the authority to issue a regulation
   purporting to define the term as such.
          Cargill is correct. A plain reading of the statutory language, paired
   with close consideration of the mechanics of a semi-automatic firearm, re-
   veals that a bump stock is excluded from the technical definition of “ma-
   chinegun” set forth in the Gun Control Act and National Firearms Act.
          But even if that conclusion were incorrect, the rule of lenity would still
   require us to interpret the statute against imposing criminal liability. A rich
   legal tradition supports the “well known rule” that “penal laws are to be con-
   strued strictly.” United States v. Wiltberger, 18 U.S. (5 Wheat.) 76, 94–95
   (1820). As Chief Justice Marshall explained long ago, the rule “is founded
   on the tenderness of the law for the rights of individuals; and on the plain
   principle that the power of punishment is vested in the legislative, not in the
   judicial department. It is the legislature, not the Court, which is to define a
   crime, and ordain its punishment.” Id. at 95.
          The Government’s regulation violates these principles. As an initial
   matter, it purports to allow ATF—rather than Congress—to set forth the

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   scope of criminal prohibitions. Indeed, the Government would outlaw bump
   stocks by administrative fiat even though the very same agency routinely in-
   terpreted the ban on machineguns as not applying to the type of bump stocks
   at issue here. Nor can we say that the statutory definition unambiguously sup-
   ports the Government’s interpretation. As noted above, we conclude that it
   unambiguously does not. But even if we are wrong, the statute is at least am-
   biguous in this regard. And if the statute is ambiguous, Congress must cure
   that ambiguity, not the federal courts.
          The definition of “machinegun” as set forth in the National Firearms
   Act and Gun Control Act does not apply to bump stocks. And if there were
   any doubt as to this conclusion, we conclude that the statutory definition is
   ambiguous, at the very least. The rule of lenity therefore compels us to con-
   strue the statute in Cargill’s favor. Either way, we must REVERSE.
                                          I
                                         A
          The Gun Control Act of 1968 provides that “it shall be unlawful for
   any person to transfer or possess a machinegun.” 18 U.S.C. § 922(o)(1). The
   Act defines machinegun as follows:
          The term “machinegun” means any weapon which shoots, is
          designed to shoot, or can be readily restored to shoot, automat-
          ically more than one shot, without manual reloading, by a single
          function of the trigger. The term shall also include the frame or
          receiver of any such weapon, any part designed and intended
          solely and exclusively, or combination of parts designed and in-
          tended, for use in converting a weapon into a machinegun, and
          any combination of parts from which a machinegun can be as-
          sembled if such parts are in the possession or under the control
          of a person.

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   26 U.S.C. § 5845(b); see 18 U.S.C. § 921(a)(24) (incorporating the definition
   in the National Firearms Act).
          The traditional example of a machinegun 1 is a rifle capable of auto-
   matic fire, like the M-16. Semi-automatic rifles like the AR-15 are not ma-
   chineguns. See Hollins v. Lynch, 827 F.3d 436, 440 n.2 (5th Cir. 2016) (The
   M-16 “is capable of automatic fire, that is to say, firing more than one round
   per trigger-action. . . . The AR-15 is essentially a semi-automatic version of
   the M-16, that is to say, it fires only one round per trigger-action.”).
                                             B
          To understand what a machinegun is, it is helpful to understand what
   a machinegun is not. To that end, the firing mechanism of a semi-automatic
   weapon is especially important. The relevant parts are as follows:

          The trigger is the interface between the gun’s internal mechanism and
   the human finger. The sear is the trigger’s top-forward geometric plane,
   which locks snugly into a groove near the spring of the hammer. The hammer
   is the spring-loaded element that strikes the firing pin, causing ignition of the

          1
            We spell machinegun as being one word because that is how Congress has defined
   the term in the statutes at issue here.

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   charge and propulsion of the bullet. The disconnector is a part that sits on
   top of the trigger and serves to reset the hammer after a round is fired; this
   resetting is what makes a semi-automatic weapon semi-automatic.
           The mechanics of the firing process are as follows. First, the user pulls
   the trigger. Doing so disengages the hammer from the sear, allowing the
   spring to swing the hammer to strike the firing pin, which causes the charge
   to combust and propel the bullet. The firing of the bullet thrusts the bolt
   backward, which kicks the hammer into the disconnector on top of the still-
   depressed trigger. When the trigger is reset, the hammer is pulled back into
   the cocked position and secured by the trigger’s sear as it slips off the discon-
   nector. The user may then fire again by pulling the trigger, without having to
   manually re-cock the hammer. The mechanics are viewed below: 2

          The end result is that the user of a semi-automatic firearm can fire
   rapidly by means of repeated use of the trigger. Critically, use of the trigger
   necessarily corresponds one-to-one with bullets fired. That is, a single pull
   of the trigger results in a single bullet fired. Without resetting the trigger, the
   disconnector cannot reset the hammer to the fully cocked position. And

          2
             This figure is a stationary image taken from an animated graphic that moves to
   display the relevant motion. The moving image may be found here: https://www.ca5.
   uscourts.gov/opinions/pub/20/20-51016_ar15.gif

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   unless the hammer is fully cocked, it will not be able to strike the firing pin
   with sufficient force to discharge the weapon a second time. (When a weapon
   fails to fire for this reason, it is said to experience a “hammer follow” mal-
   function.) In sum, both the hammer and the trigger-disconnector must in-
   variably return full circle before another round can be dispatched.
          This process may be contrasted with a fully automatic gun, which is
   equipped with something called an “auto sear”—a device that serves to re-
   cock and release the hammer in tandem with the motion of the bolt for so
   long as the trigger remains depressed. In other words, the auto sear enables
   a pendulum swing of the hammer in sync with the bolt without any further
   input from the user; with one pull of the trigger, an automatic weapon can
   shoot continuously until ammunition is depleted.

                                         C
          The statutory definition of a machinegun also includes devices that
   convert an ordinary firearm into a machinegun. See United States v. Camp,
   343 F.3d 743 (5th Cir. 2003) (holding that a switch—which, if flipped, would
   cause a semi-automatic rifle to fire continuously—is a device that turns an
   ordinary firearm into a machinegun). The issue presented here is whether a
   bump stock is such a device.
          A bump stock is an accessory that attaches to a semi-automatic
   weapon and assists the shooter to engage in bump firing. Bump firing, in turn,
   is a technique whereby a shooter uses a firearm’s natural recoil to quickly
   reengage the trigger, resulting in an increased rate of fire. It is possible to
   bump fire an ordinary semi-automatic rifle without any assisting device, but
   a bump stock makes the technique easier.
          A typical bump stock consists of a sliding shoulder stock molded to a
   grip, a trigger ledge where the shooter places his finger, and a detachable

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   rectangular receiver module that goes into the receiver well of the bump
   stock’s handle to guide the recoil of the weapon when fired. To begin bump
   firing, the shooter presses forward on the firearm’s forebody to bump into
   the trigger finger. The gun then slides back and forth, and the recoil energy
   forces the gun backward, re-engaging the trigger. The shooter maintains for-
   ward pressure on the gun’s forebody, again causing the trigger to bump into
   the trigger finger, maintaining fire. The firing process may be viewed as fol-
   lows: 3

             In summary, a bump stock combines with a semi-automatic weapon to
   facilitate the repeated function of the trigger. To be sure, it makes the pro-
   cess faster and easier. But the mechanics remain exactly the same: the firing
   of each and every round requires an intervening function of the trigger. This
   does not alter the form of manual input that the user must provide to dis-
   charge the weapon. Without a bump stock or the use of an alternative bump
   technique, the user must provide manual input by pulling the trigger with the
   muscles of his trigger finger. With a bump stock, the shooter need not pull

             3
             This figure is a stationary image taken from an animated graphic that moves to
   display the relevant motion. The moving image may be found here: https://www.ca5.
   uscourts.gov/opinions/pub/20/20-51016_bump_fire_animation.gif

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   and release his trigger finger. But the shooter must still apply forward pres-
   sure to the weapon’s forebody in order to maintain the shooting mechanism.
   Again, the manual input remains, even though its form changes.
          We note one important distinction. Some bump stocks—called me-
   chanical bump stocks—are equipped with springs or other internal mechan-
   ical devices that automatically assist the shooter to engage in bump firing.
   For such a bump stock, the shooter does not have to maintain pressure on the
   barrel and trigger ledge in order to maintain this firing sequence. Only non-
   mechanical bump stocks are at issue in this case.
                                          D
          Bump stocks were first invented in the early 2000s. Historically, ATF
   distinguished between mechanical and non-mechanical bump stocks in cate-
   gorizing a particular accessory as a machinegun. This categorization is done
   through ATF’s Firearms Technology Branch, which is authorized to issue
   classification letters upon request from members of the public. See 26 U.S.C.
   § 5841(c) (requiring firearm manufacturers and possessor to receive “author-
   ization”); Bureau of Alcohol, Tobacco, Firearms and Explosives, National
   Firearms Handbook § 7.2.4 (setting forth the classification process). When
   ATF first considered mechanical bump stocks in 2006, it categorized them
   as machineguns: “[A] device attached to a semiautomatic firearm that uses
   an internal spring to harness the force of a firearm’s recoil so that the firearm
   shoots more than one shot with a single pull of the trigger is a machinegun.”
   83 Fed. Reg. at 66514. ATF maintains that categorization.
          But from the time ATF first considered non-mechanical bump stocks
   to 2017, it categorized that type of bump stock as not being a machinegun. In
   that time, the Firearms Technology Branch issued dozens of classification
   letters regarding non-mechanical bump stocks, each time arriving at the same
   conclusion. One letter from 2010 is illustrative:

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           Dear [Applicant], This is in reference to your submission . . .
           asking for an evaluation of a replacement shoulder stock for an
           AR-15 type rifle. Your letter advises that the stock (referenced
           in this reply as a “bump-stock”) is intended to assist persons
           whose hands have limited mobility to “bump-fire” an AR-15
           type rifle. . . . The stock has no automatically functioning me-
           chanical parts or springs and performs no automatic mechani-
           cal function when installed. In order to use the installed device,
           the shooter must apply constant forward pressure with the
           non-shooting hand and constant rearward pressure with the
           shooting hand. Accordingly, we find that the “bump-stock” is
           a firearm part and is not regulated as a firearm under the Gun
           Control Act or the National Firearms Act.

           However, bump-stock classification reached a point of inflection on
   October 1, 2017. On that day, a gunman murdered over 50 innocent men and
   women in Las Vegas, and injured 500 more. He used several weapons, many
   of which were equipped with extended magazines and bump stocks. These
   tragic events thrust bump stocks into the center of national attention.
           Within ten days of the shooting, two bills prohibiting bump-stock de-
   vices were proposed in Congress. See Automatic Gunfire Prevention Act, H.R.
   3947, 115th Cong. (2017) 4; To Amend Title 18, United States Code, To Prohibit
   the Manufacture, Possession, or Transfer of Any Part or Combination of Parts
   That is Designed and Functions to Increase the Rate of Fire of a Semi-automatic
   Rifle, H.R. 3999, 115th Cong. (2017). While Congress debated the bills, ATF
   published a notice of proposed rulemaking, intending to reverse its previous
   interpretation that non-mechanical bump stocks are not machineguns for
   purposes of the National Firearms Act and Gun Control Act. Notice of Pro-
   posed Rulemaking, Bump-Stock-Type Devices, 83 Fed. Reg. 13442 (Mar. 29,

           4
             An identical bill was proposed in the Senate. Automatic Gunfire Prevention Act, S.
   1916, 115th Cong. (2017).

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   2018). Senator Diane Feinstein—who sponsored one of the bills mentioned
   above—expressed concern with the proposed rule:
           Until today, the ATF has consistently stated that bump stocks
           could not be banned through regulation because they do not fall
           under the legal definition of a machine gun.
           Now, the department has done an about face, claiming that
           bump stocks do fall under the legal definition of a machine gun
           and it can ban them through regulations. The fact that ATF
           said as recently as April 2017 that it lacks this authority gives
           the gun lobby and its allies even more reason to file a lawsuit to
           block the regulations.
          Unbelievably, the regulation hinges on a dubious analysis
          claiming that bumping the trigger is not the same as pulling it.
          The gun lobby and manufacturers will have a field day with this
          reasoning. What’s more, the regulation does not ban all devices
          that accelerate a semi-automatic weapon[’]s rate of fire to that
          of a machine gun.
          Both Justice Department and ATF lawyers know that legisla-
          tion is the only way to ban bump stocks. The law has not
          changed since 1986, and it must be amended to cover bump
          stocks and other dangerous devices like trigger cranks. Our bill
          does this—the regulation does not.

   Press Release, Sen. Dianne Feinstein, Feinstein Statement on Regulation to
   Ban Bump Stocks (Mar. 23, 2018). ATF continued with the rulemaking pro-
   cess, publishing the final rule later that year. Final Rule, Bump-Stock-Type
   Devices, 83 Fed. Reg. 66514 (Dec. 26, 2018). 5 The Final Rule purported to
   modify the definition of machinegun as follows:

          5
             Acting Attorney General Matthew Whitaker initially signed the Final Rule, but
   some questioned his authority to do so. In response, Attorney General William Barr rati-
   fied the Final Rule upon his being sworn into office. 84 Fed. Reg. 9239 (Mar. 14, 2019).

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         A “machinegun,” “machine pistol,” “submachinegun,” or
         “automatic rifle” is a firearm which shoots, is designed to
         shoot, or can be readily restored to shoot, automatically more
         than one shot, without manual reloading, by a single function
         of the trigger. The term shall also include the frame or receiver
         of any such weapon, any part designed and intended solely and
         exclusively, or combination of parts designed and intended, for
         use in converting a weapon into a machinegun, and any combi-
         nation of parts from which a machinegun can be assembled if
         such parts are in the possession or under the control of a per-
         son. For purposes of this definition, the term “automatically”
         as it modifies “shoots, is designed to shoot, or can be readily
         restored to shoot,” means functioning as the result of a self-
         acting or self-regulating mechanism that allows the firing of
         multiple rounds through a single function of the trigger; and
         “single function of the trigger” means a single pull of the trig-
         ger and analogous motions. The term “machinegun” includes
         a bump-stock-type device, i.e., a device that allows a semi-au-
         tomatic firearm to shoot more than one shot with a single pull
         of the trigger by harnessing the recoil energy of the semi-auto-
         matic firearm to which it is affixed so that the trigger resets and
         continues firing without additional physical manipulation of
         the trigger by the shooter.

   83 Fed. Reg. at 66553–54.

                                         E
         Plaintiff Michael Cargill lawfully acquired two non-mechanical bump
   stocks but surrendered them to ATF after passage of the Final Rule. He then
   sued ATF and other federal defendants, bringing several claims under the
   Administrative Procedure Act. First, he contends that ATF lacked authority
   to promulgate the Final Rule because its interpretation of machinegun con-
   flicts with the unambiguous statutory definition. And even if the statute is
   ambiguous, Cargill says, it should be construed in his favor because of the

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   rule of lenity. And because the statute concerns criminal penalties, the Gov-
   ernment’s interpretation is not entitled to deference under Chevron USA,
   Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837 (1984). Cargill
   also argues that the Final Rule constitutes an unconstitutional exercise of leg-
   islative power by an administrative agency.
          After a one-day bench trial, the district court entered judgment for the
   Government. Cargill v. Barr, 502 F. Supp. 3d 1163 (W.D. Tex. 2020). It
   declined to apply Chevron but found that the Government’s new interpreta-
   tion of machinegun is the best reading of the relevant statute. The district
   court also rejected Cargill’s nondelegation claim. A panel of this court af-
   firmed, concluding that the Final Rule’s interpretation of machinegun is the
   best reading of the statute, and declining to reach Chevron or the nondelega-
   tion question. Cargill v. Garland, 20 F.4th 1004 (5th Cir. 2021). We granted
   rehearing en banc, vacating the panel opinion. 37 F.4th 1091.
                                          II
          Three of our sister circuits have reviewed preliminary-injunction mo-
   tions relating to the Final Rule. The issues engendered great disagreement,
   but each circuit that has addressed them agrees that the definition of ma-
   chinegun within the National Firearms Act and Gun Control Act does not
   unambiguously mean what the Government says it means. Each circuit ulti-
   mately denied preliminary injunctive relief, and the Supreme Court denied
   each of the certiorari petitions. See, e.g., Guedes v. ATF, 140 S. Ct. 789, 791
   (2020) (Gorsuch, J., statement respecting denial of certiorari) (“Despite
   these concerns, I agree with my colleagues that the interlocutory petition be-
   fore us does not merit review.”).
          A divided D.C. Circuit panel determined that the Final Rule is ambig-
   uous, but applied Chevron deference to the Government’s statutory interpre-
   tation. Guedes v. ATF, 920 F.3d 1 (D.C. Cir. 2019). The dissent contended

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   that the Final Rule contradicts the statute’s plain meaning. Id. at 35 (Hen-
   derson, J., concurring in part and dissenting in part). Although the Supreme
   Court denied certiorari, 140 S. Ct. 789 (2020), one Justice wrote separately
   to explain his view that Chevron does not apply because (i) the Government
   had expressly waived its application, (ii) it does not apply to regulations bear-
   ing criminal sanctions, and (iii) the Final Rule directly contradicts the Gov-
   ernment’s previous interpretation. Id. at 789–91 (Gorsuch, J., statement re-
   specting denial of certiorari).
          In another divided opinion, the Tenth Circuit reached the same con-
   clusion as the D.C. Circuit: that the Final Rule was ambiguous and entitled
   to Chevron deference. Aposhian v. Barr, 958 F.3d 969 (10th Cir. 2020); see id.
   at 991 (Carson, J., dissenting). The court initially granted rehearing en banc,
   vacating the panel decision, 973 F.3d 1151 (10th Cir. 2020) (en banc), but
   later vacated the order as improvidently granted, reinstating the former opin-
   ion. Aposhian v. Wilkinson, 989 F.3d 890 (10th Cir. 2021) (en banc), cert de-
   nied sub nom. Aposhian v. Garland, 143 S. Ct. 83 (2022). Three dissents were
   written, each of which was joined by five of the eleven participating judges.
   The first dissent would have held that (i) the statute unambiguously does not
   apply to bump stocks, (ii) Chevron does not apply either because the Govern-
   ment waived it or because it does not apply in the criminal context, and (iii)
   the Final Rule may violate nondelegation principles. Id. at 891–903 (Tym-
   kovich, C.J., dissenting); see also id. at 903–04 (Hartz, J., dissenting); id. at
   904–06 (Eid, J., dissenting); id. at 906–08 (Carson, J., dissenting).
          Finally, in yet another divided opinion, a Sixth Circuit panel ruled
   against the Government, declining to apply Chevron deference and holding
   the statutory definition of machinegun does not include bump stocks. Gun
   Owners of America, Inc. v. Garland, 992 F.3d 446, 450 (6th Cir. 2021); but see
   id. at 475-92 (White, J., dissenting) (arguing that Chevron applies and that the
   Government’s interpretation is reasonable). The court granted rehearing en

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   banc, 2 F.4th 576 (6th Cir. 2021) (en banc), and an evenly-divided court af-
   firmed the district court’s denial of the preliminary injunction. 19 F.4th 890
   (6th Cir. 2020), cert denied, 143 S. Ct. 83 (2022).
          In addition to these circuit decisions, the Navy-Marine Corps Court
   of Criminal Appeals considered the Final Rule in the context of a criminal
   prosecution for possession of a machinegun. See United States v. Alkazahg,
   81 M.J. 764, 780–81 (N–M. Ct. Crim. App. 2021). That court determined
   that, under the best reading of the statutory language, a bump stock is not a
   machinegun. But it ultimately found that the statute is ambiguous and ap-
   plied the rule of lenity to construe the statute against imposing criminal lia-
   bility. It dismissed the charge for possession of a machinegun.
                                         III
          Our primary task is to interpret the meaning of machinegun as defined
   in 26 U.S.C. § 5845(b). Of course, the Government has sponsored its own
   interpretation, as expressed in the Final Rule. Ordinarily, that action would
   invoke the two-step Chevron framework. As we recently summarized, “[a]t
   step one, we ask whether Congress has directly spoken to the precise ques-
   tion at issue, in which case we must give effect to the unambiguously ex-
   pressed intent of Congress and reverse an agency’s interpretation that fails
   to conform to the statutory text.” Huawei Technologies USA, Inc. v. FCC, 2
   F.4th 421, 433 (5th Cir. 2021) (quotations omitted).
          But here, the Government has declined to invoke Chevron in any of
   the lawsuits challenging the Final Rule. Nonetheless, several circuits have
   applied Chevron deference to challenges of the Final Rule, and so we will con-
   sider Chevron’s applicability below. But before we do, we determine the stat-
   ute’s meaning using traditional statutory-interpretation tools. That is, “the
   old-fashioned way.” Guedes, 140 S. Ct. at 790 (Gorsuch, J., statement re-
   specting denial of certiorari); see also Guedes, 920 F.3d at 42 (Henderson, J.,

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   concurring in part and dissenting in part); Aposhian, 989 F.3d at 898 (Tym-
   kovich, C.J., dissenting). If the statute is unambiguous, it does not matter
   whether Chevron applies. See, e.g., Western Refining Southwest, Inc. v. FERC,
   636 F.3d 719, 727 (5th Cir. 2011) (“[If] the statute’s text is unambiguous, we
   need not proceed to Step Two of Chevron.”).
           Recall that in this circumstance, Congress has defined machinegun to
   mean “any weapon which shoots . . . automatically more than one shot . . . by
   a single function of the trigger,” or any accessory that allows a firearm to
   shoot in that manner. 26 U.S.C. § 5845(b). The parties dispute whether the
   fire created by a semi-automatic rifle equipped with a non-mechanical bump
   stock is produced both “automatically” and “by single function of the trig-
   ger.” We address those components in reverse order. 6
                                                 A
           The first phrase we consider is “by a single function of the trigger.”
   At the time the statute was passed, “function” meant “action.” Webster’s
   New International Dictionary 1019 (2d ed. 1934); see Guedes, 920 F.3d at 43
   (Henderson, J., concurring in part and dissenting in part); Aposhian, 989 F.3d
   at 895 (Tymkovich, C.J., dissenting). Thus, the relevant question is whether

           6
              Cargill also argues that the Final Rule is void because it is a legislative rule—as
   opposed to an interpretive rule—and because relevant federal law does not authorize ATF
   to issue such a rule. We assume arguendo that the Final Rule is legislative in nature and that
   ATF is authorized to issue such a rule. First, if the rule were interpretive in nature, it would
   not be eligible for Chevron deference. United States v. Mead Corp., 533 U.S. 218, 232 (2001).
   But as explained infra, three independent reasons demand that we not defer to the Govern-
   ment here. Second, if ATF were not authorized to promulgate legislative rules, the rule at
   issue would be void. Ultimately, however, we conclude that the Government’s interpreta-
   tion is inconsistent with the statutory definition, so ATF lacked authority to issue the Final
   Rule. We therefore need not consider Cargill’s additional argument that the Final Rule is
   a legislative rule.

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   a semi-automatic rifle equipped with a non-mechanical bump stock fires
   more than one shot each time the trigger “acts.”
          It does not. As illustrated above, a semi-automatic weapon utilizes a
   simple mechanical process: the trigger disengages the hammer from the sear,
   the hammer strikes the firing pin, the bullet fires, and the recoil pushes the
   hammer against the disconnector, which resets the trigger. This process hap-
   pens every single time one bullet is fired. To be sure, a non-mechanical bump
   stock increases the rate at which the process occurs. But the fact remains
   that only one bullet is fired each time the shooter pulls the trigger.
          The Government contends that “single function of the trigger”
   means “a single pull of the trigger and analogous movements.” 83 Fed. Reg.
   at 66553. That is, according to the Government, “function” means “pull.”
   But that argument fails on its face because a shooter still pulls the trigger of a
   semi-automatic weapon equipped with a non-mechanical bump stock each
   time he or she fires a bullet. Without a bump stock, the trigger activates be-
   cause the shooter flexes his or her finger; with a bump stock, the trigger acti-
   vates because the recoil of the previous shot re-engages the trigger and the
   shooter’s maintained force on the gun’s forebody bumps the trigger against
   the shooter’s finger. This is a distinction without a difference—the end re-
   sult in both cases is that the trigger is pulled. See Guedes, 920 F.3d at 48
   (Henderson, J., concurring in part and dissenting in part) (“A semiautomatic
   rifle shoots a single round per pull of the trigger and the bump stock changes
   only how the pull is accomplished.”); Gun Owners of America, 992 F.3d at
   469–73, vacated, 2 F.4th 576. Even if “single function” meant “single pull,”
   the definition would still not include a non-mechanical bump stock. Moreo-
   ver, even though pulling the trigger can sometimes begin the bump firing se-
   quence, the process is more typically begun by pushing forward on the fore-
   body of the firearm.

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          For several of our sister circuits, however, the plain language is not so
   plain. They reason that single function of the trigger “could mean ‘a single
   pull of the trigger from the perspective of the shooter.’” Guedes, 920 F.3d at
   29; see also Gun Owners of America, 19 F.4th at 905 (White, J., in support of
   affirmance). Considering the definition of “function,” one court understood
   the issue as such: “[T]hat definition begs the question of whether ‘function’
   requires our focus upon the movement of the trigger, or the movement of the
   trigger finger. The statute is silent in this regard.” Aposhian, 958 F.3d at 986.
   According to that logic, for a semi-automatic rifle equipped with a non-me-
   chanical bump stock, the act of pulling the trigger—which begins the bump
   firing sequence—is a single pull for purposes of the Gun Control Act and
   National Firearms Act.
          The problem with that interpretation is that it is based on words that
   do not exist in the statute. The statute “uses ‘single function of the trigger,’
   not single function of the shooter’s trigger finger.” Guedes, 920 F.3d at 48
   (Henderson, J., concurring in part and dissenting in part); see also Aposhian,
   989 F.3d at 895 (Tymkovich, C.J., dissenting) (“The statute speaks only to
   how the trigger acts, making no mention of the shooter.”). The Navy-Marine
   Corps Court of Appeals likewise refused to read words into the statute:
          The best read implies that the shooter initiates the trigger func-
          tion by some action, such as pulling the trigger—or it could be
          by just pushing a button—and it is the follow-on action where
          the trigger acts out its mechanical design or purpose that
          speaks to the “function of the trigger.” The statute does not
          say “by a single function of the trigger finger” nor does it say
          “by a single pull of the trigger in addition to external pressure
          from the shooter’s non-firing hand.” . . . . Had Congress
          wanted to use the phrase “by a single pull of the trigger” for
          machine guns, it could have. But it did not.

   Alkazahg, 81 M.J. at 780–81.

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          We agree. The statutory definition of machinegun utilizes a grammat-
   ical construction that ties the definition to the movement of the trigger itself,
   and not the movement of a trigger finger. Nor do we rely on grammar alone.
   Context firmly corroborates what grammar initially suggests by demonstrat-
   ing that Congress knew how to write a definition that is keyed to the move-
   ment of the trigger finger if it wanted to. But it did not. The Government
   offers nothing to overcome this plain reading, so that we are obliged to con-
   clude that the statutory definition of machinegun unambiguously turns on the
   movement of the trigger and not a trigger finger.
           Grammar rejects a reading based on the shooter’s perspective. Each
   component of the statutory definition supports the mechanical perspective,
   not a shooter’s perspective.        Again, the definition reads as follows:
   “[M]achinegun means . . . any weapon which shoots . . . automatically more
   than one shot . . . by a single function of the trigger.” 26 U.S.C. § 5845(b).
   The subject of the sentence, of course, is machinegun. The linking verb means
   connects the subject to the subject complement—weapon. Next, the adjec-
   tival phrase which shoots modifies weapon. The adverbial phrase automatically
   more than one shot then modifies shoots. Finally, two prepositional phrases
   follow. The first, by a single function, modifies the adverbial phrase. The sec-
   ond, of the trigger, modifies the first prepositional phrase. See also Guedes, 920
   F.3d at 44 n.13 (Henderson, J., concurring in part and dissenting in part) (di-
   agramming the statutory definition).
          The first thing to note is that the ultimate subject is machinegun, and
   the subject complement is weapon. In other words, a machinegun is defined
   by reference to what kind of weapon it is. But identifying the subject of the
   sentence is only our first step. We next look, second, to the fact that the term
   weapon is defined by how it shoots. So, again, the definition refers to the de-
   vice being made to shoot, not the person or thing doing the shooting. Third,
   the manner of shooting must be automatic. Fourth—and critically—the

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   prepositional phrases define the firing process’s requirements from a me-
   chanical perspective. The process must occur by a single function, and the
   single act must be by the trigger. In short, there is no mention of a shooter.
   The grammatical structure continuously points the reader back to the me-
   chanics of the firearm. The statute does not care what human input is re-
   quired to activate the trigger—it cares only whether more than one shot is
   fired each time the trigger acts.
          We do not stop with the grammar. With statutes, “[c]ontext is a pri-
   mary determinant of meaning. Antonin Scalia & Bryan A. Garner, Reading
   Law: The Interpretation of Legal Texts 167 (2012). So, we look next to context
   for further clues. And context confirms that the statute must be read from
   the mechanical perspective. Specifically, context tells us that Congress knew
   how to write a definition that explicitly turns on the action of a shooter rather
   than the action of a trigger, but chose not to do so here. Immediately follow-
   ing the definition of machinegun provided in 26 U.S.C. § 5845(b), Congress
   defined the term “rifle” to mean a weapon designed “to use the energy of
   the explosive in a fixed cartridge to fire only a single projectile through a rifled
   bore for each single pull of the trigger.” § 5845(c) (emphasis added). The stat-
   ute next defines “shotgun” to mean a weapon designed “to use the energy
   of the explosive in a fixed shotgun shell to fire through a smooth bore either
   a number of projectiles (ball shot) or a single projectile for each pull of the trig-
   ger.” Id. § 5845(d) (emphases added). “[W]here the document has used one
   term in one place, and a materially different term in another, the presumption
   is that the different term denotes a different idea.” Reading Law at 170.
          To summarize, the definition of machinegun must turn on the action
   (or “function”) of the trigger because no other actor is mentioned or implied.
   This conclusion is only strengthened by the fact that other definitions within
   the same statutory provision explicitly turn on the action of a shooter, show-
   ing that Congress knew how to write a definition that proceeds from a

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   shooter’s perspective, rather than a mechanical one, if it had wanted to. The
   notion that the definition turns on the action of an unnamed shooter is incon-
   sistent with both the grammatical and statutory contexts.
           The Government says that this straightforward interpretation defies
   common sense. It would not have been prudent for Congress to “zero[] in
   on the mechanistic movement of the trigger,” the Government says, because
   the problem sought to be remedied was “the ability to drastically increase a
   weapon’s rate of fire.” Aposhian v. Barr, 374 F. Supp. 3d 1145, 1152 (D. Utah
   2019)). Perhaps Congress’s choice of words was prudent, or perhaps it was
   not. That is not for us to decide. But the Government’s objection only ac-
   centuates the fact that Congress did not use words describing the shooter’s
   perspective or the weapon’s rate of fire. See Alkazahg, 81 M.J. at 781 (“Con-
   gress could have suggested that a shooter-focused approach or even a rate-
   of-fire approach was the way to read the statute by enacting those words.
   Even the term ‘machine gun’ suggests a mechanical approach where the
   shooter interaction is extremely limited.”). Instead, it made up an entirely
   new phrase—by a single function of the trigger—that specifically pertains to
   the mechanics of a firearm. Prudent or not, Congress defined the term “ma-
   chinegun” by reference to the trigger’s mechanics. We are bound to apply
   that definition as written. 7

           7
             Although our reasoning is independently sufficient to support our conclusion, the
   application of corpus linguistics only provides further support. A search in the Corpus of
   Historical American English, which contains more than 100,000 individual texts from the
   1820s–2010s and more than 475 million words, shows zero usage of the phrase “function
   of the trigger,” “function of a trigger,” “function of triggers,” or “function of the trig-
   gers.” Corpus of Historical American English, English Corpora, https://www.english-cor-
   pora.org/coha/. Similarly, a search in the News on the Web Corpus (NOW Corpus),
   which contains more than 16 billion words from more than 27 million online texts from
   2010 to present day, shows only 24 uses of the phrase “function of the trigger”—all of

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           The Government also points to our decision in United States v. Camp,
   arguing that it controls here. 343 F.3d 734. It does not. But to the extent
   Camp applies, it supports Cargill’s position. The issue presented in Camp
   was whether something other than the metal lever that ordinarily begins the
   firing process can be a “trigger” for purposes of the Gun Control Act and
   National Firearms Act. The defendant there modified a semi-automatic rifle,
   building a switch behind the original trigger that, when pulled, “supplied
   electrical power to a motor connected to the bottom of a fishing reel that had
   been placed inside the weapon’s trigger guard; the motor caused the reel to
   rotate; and that rotation caused the original trigger to function in rapid suc-
   cession.” 343 F.3d at 744. We held that the weapon was a machinegun even
   though the gun’s original trigger activated each time a bullet was fired. That
   was so because the gun had been modified such that it had a new trigger. As
   we held in United States v. Jokel, 969 F.2d 132, 135 (5th Cir. 1992), a trigger
   is just the “mechanism . . . used to initiate the firing sequence.” The mech-
   anism used to initiate the firing sequence in Camp was the new switch. That
   switch operated by a single function, and so the firearm met the statutory
   definition of a machinegun. Camp, 343 F.3d at 745.
           Here, no party cites Camp for the proposition that the legally relevant
   trigger is anything other than the traditional trigger. And for good reason.
   All a non-mechanical bump stock does is allow the shooter to fire at an in-
   creased rate by harnessing a weapon’s natural recoil to re-engage the trigger,

   which are from news sources directly quoting a firearm statute; there were zero uses of the
   phrase “function of a trigger,” “function of triggers,” or “function of the triggers.” NOW
   Corpus, English Corpora, https://www.english-corpora.org/now/. The upshot is that the
   phrase “by a single function of the trigger” is a novel phrase created by Congress specifi-
   cally to be used in these firearm statutes. The Government stresses the existence of other
   ordinary phrases, like “pull of the trigger.” Perhaps these ordinary phrases exist, but Con-
   gress did not use them here.

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                                          No. 20-51016

   and by using the shooter’s maintained forward force. The case might well be
   different if we were considering a semi-automatic weapon equipped with a
   mechanical bump stock. It could be the case that a switch activating a me-
   chanical bump stock would be the legal trigger. But we are not considering
   that case. 8 Here, the definition of “trigger” is not in dispute. If anything,
   Camp supports our conclusion because the trigger at issue in this case is dif-
   ferent in kind from the trigger at issue there.
                                                B
           Even if a non-mechanical bump stock caused a semi-automatic rifle to
   operate by a single function of the trigger, the rifle would still need to operate
   automatically in order to be a machinegun. 9 All generally agree that here,
   automatically means “self-acting.” Oxford English Dictionary at 574 (“[s]elf-
   acting under conditions fixed for it, going of itself”); see also Cargill, 20 F.4th
   at 1012; Guedes, 920 F.3d at 30; id. at 43 (Henderson, J. concurring in part
   and dissenting in part); Aposhian, 958 F.3d at 986; Aposhian, 989 F.3d at 895
   (Tymkovich, C.J., dissenting); Gun Owners of America, 19 F.4th at 905–06

           8
             The Government points to an unpublished Eleventh Circuit case—which held
   that a mechanical bump stock called the Akins Accelerator is a machinegun for purposes of
   federal law—as support for its argument that non-mechanical bump stocks operate by a
   single function of the trigger. See Akins v. United States, 312 F. App’x 197, 200 (11th Cir.
   2009). But that evidence cuts in the other direction. Unlike nonmechanical bump stocks,
   a shooter using an Akins Accelerator need only pull the trigger once to activate the firing
   sequence. The mechanical bump stock then maintained the bump fire of its own accord.
   See 83 Fed. Reg. at 66517. Precisely for that reason, our decision today would not apply to
   an Akins Accelerator.
           9
             As explained above, because a semi-automatic firearm equipped with a non-me-
   chanical bump stock does not operate “by a single function of the trigger,” such a weapon
   is not a machinegun, and we must render judgment for Cargill. Our conclusions in Parts
   III.B and V are each independent, alternative holdings. In the Fifth Circuit, “alternative
   holdings are binding precedent and not obiter dictum.” Jarkesy v. SEC, 34 F.4th 446, 459
   n.9 (5th Cir. 2022) (quotation omitted).

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                                       No. 20-51016

   (White, J., in support of affirmance); id. at 912–13 (Murphy, J., in opposition
   to affirmance). But the parties dispute whether the firing process enabled by
   a non-mechanical bump stock is self-acting.
          It is not. As an initial matter, we must remember that the phrase “by
   a single function of the trigger” modifies the adverb “automatically.” Thus,
   the condition is satisfied only if it is the trigger that causes the firearm to shoot
   automatically. See Guedes, 920 F.3d at 43 (Henderson, J., concurring in part
   and dissenting in part) (‘“Automatically’ cannot be read in isolation. On the
   contrary, it is modified—that is, limited—by the clause ‘by a single function
   of the trigger.’”); Aposhian, 989 F.3d at 896 (Tymkovich, C.J., dissenting).
   That is not how a bump stock works. Bump firing does not maintain if all a
   shooter does is initially pull the trigger. Rather, to continue the firing after
   the shooter pulls the trigger, he or she must maintain manual, forward pres-
   sure on the barrel and manual, backward pressure on the trigger ledge.
          The Government argues that, taken together, those actions create au-
   tomatic fire. But Cargill would prevail even if that were true because those
   actions are not “a single function of the trigger.” For example, the ATF’s
   treatment of the Ithaca Model 37 “slam fire” shotgun confirms that bump
   stocks do not enable automatic fire. With the Model 37, a shooter can pull
   the trigger once and hold it. Then, after each pump with the shooter’s non-
   trigger hand, a new shell is loaded and immediately discharged. According
   to the ATF, the Model 37 fires multiple shots by a single function of the trig-
   ger, but it does not do so automatically because the shooter must manually
   pump the shotgun with his non-trigger hand. See 83 Fed. Reg. at 66,534. By
   this same logic, a rifle equipped with a non-mechanical bump stock does not
   fire automatically because the shooter must manually apply forward pressure
   on the barrel with his or her non-trigger hand.

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                                     No. 20-51016

          The Government recognizes this logic but argues that it proves too
   much. After all, the Government says, to operate a traditional automatic ri-
   fle, the shooter must pull and hold the trigger to fire more than one round.
   No one doubts that a traditional automatic weapon is a machinegun for pur-
   poses of federal law. And so it cannot be that a process is not automatic
   simply because it requires sustained input. See Guedes, 920 F.3d at 31;
   Aposhian, 958 F.3d at 987.
          That argument makes the same mistake as before: it untethers “single
   function of the trigger” from “automatically.” Restated, the statute requires
   that a machinegun be capable of firing automatically once the trigger performs
   a single function. An automatic weapon satisfies this requirement because
   the act of pulling and holding the trigger is one function, and that function
   produces more than one shot. That force must be maintained on the trigger
   does not change this conclusion. Stated succinctly:
          [A] gun shoots automatically by a single function of the trigger
          as long as the shooter need only manually cause the trigger to
          engage in a “single” function in order to fire multiple shots . . .
          So a typical machine gun qualifies even though the shooter
          pulls the trigger and keeps it pressed down because that com-
          bined external influence still does no more than result in one
          action of the trigger.

   Gun Owners of America, 19 F.4th at 915 (Murphy, J., in opposition to affir-
   mance); see also Guedes, 920 F.3d at 44 (Henderson, J., concurring in part and
   dissenting in part) (“The statutory definition of machinegun does not in-
   clude a firearm that shoots more than one round automatically by a single pull
   of the trigger and then some (that is, by constant forward pressure with the
   non-trigger hand).”) (internal quotation marks omitted and emphasis omit-
   ted). As understood by the Navy-Marine Corps Court of Appeals:

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                                     No. 20-51016

          It is incorrect to equate the holding of the trigger in an auto-
          matic weapon with the holding of the trigger and the forward
          motion in a semi-automatic weapon equipped with a bump
          stock. That is because the former is shooting automatically by
          a single function of the trigger, while the latter is relying on an
          additional human action beyond the mechanical self-acting and
          impersonal trigger function.

   Alkazahg, 81 M.J. at 782–83.
          We reiterate that a shooter can bump fire an ordinary semi-automatic
   rifle even without a bump stock. But nobody, not even the Government, con-
   tends that semi-automatic rifles are machineguns. That concession damns
   the Government’s position. As Cargill recognizes, if ordinary bump firing
   constituted automatic fire, the Final Rule would “convert a semiautomatic
   weapon into a machinegun simply by how a marksman used the weapon.”
   That absurd result reveals the flaw in the Government’s line of reasoning.
          In addition to implying absurd results, the Government’s position is
   quite telling. It would allow the use of semi-automatic rifles, which can bump
   fire, but prohibit the use of non-mechanical bump stocks, even though there
   is no mechanical difference between the two forms of gunfire. Rather, the
   meaningful difference is that, with a non-mechanical bump stock, bump firing
   is easier and can occur at a faster rate. That is a distinction Congress certainly
   could have addressed in the National Firearms Act and Gun Control Act.
   But Congress did not prohibit machineguns according to how quickly they
   fire. It prohibited machineguns according to the way that they fire. And semi-
   automatic weapons do not fire “automatically,” even when equipped with a
   non-mechanical bump stock.

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                                     No. 20-51016

                                           ***
          The definition of machinegun as set forth in the Gun Control Act and
   National Firearms Act establishes two conditions that must obtain in order
   for a weapon to qualify. The weapon must operate “automatically” and “by
   single function of the trigger.” According to the statute’s unambiguous lan-
   guage, neither condition obtains as applied to a semi-automatic rifle equipped
   with a non-mechanical bump stock. The failure of either condition is suffi-
   cient to entitle Cargill to judgment.
                                           IV
          As introduced above, several of our sister circuits applied Chevron to
   challenges to this Final Rule, even though no party requested its application.
   Because we hold that the statute is unambiguous, Chevron deference does not
   apply even if the Chevron framework does. See Western Refining Southwest,
   636 F.3d at 727. But if the statute were ambiguous, Chevron would not apply
   for any of the three reasons explained below.
                                           A
          First, Chevron does not apply for the simple reason that the Govern-
   ment does not ask us to apply it. Indeed, the Government affirmatively ar-
   gued in the district court that Chevron deference is unwarranted. As other
   jurists have recognized in this context, that means that the Chevron argument
   has been waived—not merely forfeited. United States v. Olano, 507 U.S. 725,
   733 (1993) (“Whereas forfeiture is the failure to make the timely assertion of
   a right, waiver is ‘the intentional relinquishment or abandonment of a known
   right.’”) (quoting Johnson v. Zerbst, 304 U.S. 458, 464 (1938)); Guedes, 920
   F.3d at 21 (“To the extent Chevron treatment can be waived, we assume that
   the government’s posture in this litigation would amount to a waiver rather
   than only a forfeiture.”); Aposhian, 989 F.3d at 897 (“[W]hen a party chooses
   not to pursue a legal theory potentially available to it, we generally take the

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                                     No. 20-51016

   view that it is ‘inappropriate’ to pursue that theory in our opinions.”) (inter-
   nal citation and quotation marks omitted) (Tymkovich, C.J., dissenting).
          That would seem to be the end of the inquiry, but we recognize that
   one of our sister circuits has held that Chevron cannot be waived. Guedes, 920
   F.3d at 21–23; see also Gun Owners of America, 19 F.4th at 899 n.5 (White, J.,
   in support of affirmance). To be sure, we have never held in a published case
   that Chevron must be raised by the Government in order to apply. See Albanil
   v. Coast 2 Coast, Inc., 444 F. App’x 788, 796 (5th Cir. 2011). But the conclu-
   sion is obvious, and flows from well-settled waiver principles. After all, that
   a court should defer to the Government’s expressed interpretation is just a
   legal argument, and a party waives a legal argument if it fails to raise the ar-
   gument when presented with the opportunity. See, e.g., Sindhi v. Raina, 905
   F.3d 327, 334 (5th Cir. 2018); Fruge v. Amerisure Mut. Ins. Co., 663 F.3d 743,
   747 (5th Cir. 2011).
          As explained in the Tenth Circuit’s consideration of the Final Rule,
   “We refuse to consider arguments a party fails to make because we ‘depend
   on the adversarial process to test the issues for our decision’ and are con-
   cerned ‘for the affected parties to whom we traditionally extend notice and
   an opportunity to be heard on issues that affect them.’” Aposhian, 989 F.3d
   at 897 (Tymkovich, C.J., dissenting) (quoting Hydro Resources, Inc. v. EPA,
   608 F.3d 1131, 1146 n.10 (10th Cir. 2010) (en banc)); see also Carducci v. Regan,
   714 F.2d 171, 177 (D.C. Cir. 1983) (Scalia, J.) (“The premise of our adversar-
   ial system is that appellate courts do not sit as self-directed boards of legal
   inquiry and research, but essentially as arbiters of legal questions presented
   and argued by the parties before them.”). We must not defer to the Govern-
   ment’s interpretation here for the simple reason that no party argues that we
   should.

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                                     No. 20-51016

          If ordinary waiver principles were not enough, we note also that it
   would contradict Chevron’s central justification to defer to the Government’s
   interpretation without its urging us to do so. The justification is that ‘“policy
   choices’ should be left to executive branch officials ‘directly accountable to
   the people.”’ Guedes, 140 S. Ct. at 790 (Gorsuch, J., statement respecting
   denial of certiorari) (quoting Epic Systems v. Lewis, 138 S. Ct. 1612, 1630
   (2018) and Chevron, 467 U.S. at 865)). Here, the Government made a clear
   policy choice by declining to seek Chevron deference. The very interest un-
   derlying Chevron demands that we respect the Government’s choice and in-
   terpret the statute according to traditional principles of statutory interpreta-
   tion. See Aposhian, 989 F.3d at 898 (“If the agency disavows any reliance on
   Chevron, who are we to second-guess it?”) (Tymkovich, C.J., dissenting);
   Guedes, 140 S. Ct. at 790 (Gorsuch, J., statement respecting denial of certio-
   rari) (“[C]ourts must equally respect the Executive’s decision not to make
   policy choices in the interpretation of Congress’s handiwork.”).
          Raising the issue sua sponte, the D.C. Circuit assumed that the Gov-
   ernment’s actions were consistent with waiver, but held that Chevron cannot
   be waived. As an initial matter, that conclusion is likely inconsistent with
   Supreme Court precedent. Our highest Court “has often declined to apply
   Chevron deference when the government fails to invoke it.” Guedes, 140 S.
   Ct. at 790 (Gorsuch, J., statement respecting denial of certiorari) (collecting
   cases); see, e.g., HollyFrontier Cheyenne Refining, LLC v. Renewable Fuels
   Ass’n, 141 S. Ct. 2172, 2180 (2021) (“[T]he government is not invoking Chev-
   ron. We therefore decline to consider whether any deference might be due
   its regulation.”) (quotation omitted).
          The argument against waiver is based on the premise that Chevron is
   a standard of review. Guedes, 920 F.3d at 163; see also Gun Owners of America,
   992 F.3d at 477–78, vacated, 2 F.4th 576. It is certainly true that parties in
   litigation cannot waive the applicable standard of review. E.g., United States

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   v. Escobar, 866 F.3d 333, 339 n.13 (5th Cir. 2017) (quotation omitted). But
   Chevron is not a standard of review. The APA specifically sets forth stand-
   ards by which courts must review agency actions—arbitrary and capricious,
   abuse of discretion, in excess of statutory authority, and so on. See 5 U.S.C.
   § 706. Chevron is merely a legal argument that the Government can make to
   contend that its interpretation satisfies the relevant standard of review. 10
                                               B
           The Chevron framework does not apply for a second, independent rea-
   son: the statute which the Final Rule interprets imposes criminal penalties.
   As noted above, the primary reason for Chevron is that it allows the executive
   branch to make policy decisions through the accrued expertise of administra-
   tive agencies. But in exchange, Chevron deference shifts the responsibility
   for lawmaking from the Congress to the Executive, at least in part. That
   tradeoff cannot be justified for criminal statutes, in which the public’s enti-
   tlement to clarity in the law is at its highest. See Guedes, 140 S. Ct. at 790
   (Gorsuch, J., statement respecting denial of writ of certiorari) (“Before
   courts may send people to prison, we owe them an independent determina-
   tion that the law actually forbids their conduct.”); Esquivel-Quintana v.

           10
              The Tenth Circuit has also held that a plaintiff’s invocation of Chevron—even if
   made for the purpose of disputing the Government’s interpretation of a statute, and absent
   argument from the Government that Chevron should apply—is sufficient to require a court
   to apply the framework. Aposhian, 958 F.3d at 981–82. We respectfully disagree with that
   conclusion. Chevron’s purpose is to recognize the institutional competence of executive
   agencies and to defer to their expertise where appropriate. It would be inconsistent with
   that purpose to apply Chevron over the Government’s objection just because a plaintiff
   preemptively addresses the framework in an attempt to defend against the Government’s
   sponsored interpretation. See Aposhian, 989 F.3d at 896 (Tymkovich, C.J., dissenting)
   (“This theory of waiver is untenable. Under the panel majority’s theory, a party that chal-
   lenges an agency’s interpretation of a rule is forced to dance around Chevron, even where
   the government has not invoked it. Chevron becomes the Lord Voldemort of administrative
   law, ‘the-case-which-must-not-be-named.’”).

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   Lynch, 810 F.3d 1019, 1027 (6th Cir. 2016) (Sutton, J., concurring in part and
   dissenting in part) (Applying Chevron to criminal statutes would “permit the
   aggregation” of executive and legislative power “in the one area where its
   division matters most: the removal of citizens from society.”).
          No precedent compels Chevron’s application here. To the contrary,
   the Supreme Court has “never held that the Government’s reading of a crim-
   inal statute is entitled to any deference.” United States v. Apel, 571 U.S. 359,
   369 (2014); see also Gutierrez-Brizuela v. Lynch, 834 F.3d 1142, 1155 (10th Cir.
   2016) (“The Supreme Court has expressly instructed us not to apply Chevron
   deference when an agency seeks to interpret a criminal statute.”) (Gorsuch,
   J., concurring). That is so because “criminal laws are for the courts, not for
   the Government, to construe.” Abramski v. United States, 573 U.S. 169, 191
   (2014). For this reason, when “the Government interprets a criminal statute
   too broadly . . . or too narrowly . . . a court has an obligation to correct its er-
   ror.” Id. We must not apply Chevron where, as here, the Government seeks
   to define the scope of activities that subject the public to criminal penalties.
   This is precisely the position we have taken before, in an unpublished deci-
   sion. United States v. Garcia, 707 F. App’x 231, 234 (5th Cir. 2017) (“The
   Supreme Court has now resolved this uncertainty, instructing that no defer-
   ence is owed to agency interpretations of criminal statutes.”).
          Several of our sister circuits disagree, however. See Guedes, 920 F.3d
   at 163–67; Aposhian, 958 F.3d at 982–84. The disagreement stems from one
   paragraph in the decision Babbitt v. Sweet Home Chapter of Communities for a
   Great Oregon, 515 U.S. 687 (1995). There, the Supreme Court considered a
   Department of the Interior regulation that interpreted the Endangered Spe-
   cies Act’s criminal prohibition on “taking” an endangered species as includ-
   ing modification of the species’s habitat. The Court did not conduct a Chev-
   ron analysis, but upheld the regulation, concluding that it “owe[d] some de-
   gree of deference [to] the [DOI’s] reasonable interpretation,” in part because

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   of the “latitude the ESA gives to [DOI] in enforcing the statute.” Id. at 703.
   It also declined to apply the rule of lenity, reasoning that an administrative
   regulation does not necessarily invoke the rule just because “the governing
   statute authorizes criminal enforcement.” Id. at 704 n.18.
          Several courts cite Babbitt for the proposition that the Chevron frame-
   work applies with equal force to criminal regulations and displaces the rule of
   lenity, but it does not support that conclusion. “While Babbitt certainly cited
   Chevron and used the word deference with regard to the DOI’s interpreta-
   tion, Babbitt did not discuss or decide whether Chevron applied nor did it an-
   alyze the challenge using Chevron, just as it did not decide whether the rule
   of lenity applied or analyze the challenge using the rule of lenity.” Gun Own-
   ers of America, 992 F.3d at 457, vacated, 2 F.4th 576. As explained by two
   members of the Supreme Court, Babbitt did not purport to set forth a general
   rule respecting the interpretation of criminal regulations: “The best that one
   can say . . . is that in Babbitt[] [the Court] deferred, with scarcely any expla-
   nation, to an agency’s interpretation of a law that carried criminal penalties.
   . . . Babbitt’s drive-by ruling, in short, deserves little weight.” Whitman v.
   United States, 574 U.S. 1003, 135 S. Ct. 352, 353 (Scalia, J., joined by Thomas,
   J., respecting the denial of certiorari).
          This is confirmed by subsequent Supreme Court precedent, address-
   ing the rule of lenity in relation to Chevron and declining to defer to agency
   interpretations of criminal statutes. See Aposhian, 989 F.3d at 901 (Tym-
   kovich, C.J., dissenting) (collecting cases). Babbitt does not require us to ap-
   ply Chevron in these circumstances. Indeed, most proximate sources suggest
   otherwise. See Guedes, 140 S. Ct. at 790 (Gorsuch, J., statement respecting
   denial of certiorari) (“[W]hatever else one thinks about Chevron, it has no
   role to play when liberty is at stake.”). As such, Chevron does not apply here
   because the statutory language at issue implicates criminal penalties.

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                                                 C
           Finally, we note a third reason why Chevron deference does not apply
   in these circumstances: that ATF has adopted an interpretive position that is
   inconsistent with its prior position. To apply Chevron here would contravene
   one of the rule’s central purposes: “to promote fair notice to those subject to
   criminal laws.” United States v. Kozminski, 487 U.S. 931, 952 (1988); see also
   United States v. Kaluza, 780 F.3d 647, 669 (5th Cir. 2015) (“The rule ‘vindi-
   cates the fundamental principle that no citizen should be held accountable for
   a violation of a statute whose commands are uncertain, or subjected to pun-
   ishment that is not clearly prescribed.’”) (quoting United States v. Santos, 553
   U.S. 507, 514 (2008) (plurality op.)). 11
           If we were required to defer to the Government’s position, the Gov-
   ernment could change the scope of criminal liability at any time. Indeed, that
   is exactly what it has done here. Until 2017, the ATF had never classified
   non-mechanical bump stocks as machineguns. But now the interpretation is
   reversed, and the Government would criminalize behavior that it long recog-
   nized was lawful. In considering one of the other cases involving the regula-
   tion, one member of the Supreme Court explained the problem as such:
           Chevron’s application in this case may be doubtful for other
           reasons too. The agency used to tell everyone that bump stocks
           don’t qualify as ‘machineguns.’ Now it says the opposite. The
           law hasn’t changed, only an agency’s interpretation of it. How,
           in all this, can ordinary citizens be expected to keep up? . . .
           And why should courts, charged with the independent and

           11
              These fair-notice issues accentuate why Babbitt does not bar us from applying the
   rule of lenity. In Babbitt, the Supreme Court expressly contemplated cases where it would
   be appropriate to apply the rule of lenity. 515 U.S. at 704 n.18 (“Even if there exist regula-
   tions whose interpretations of statutory criminal penalties provide such inadequate notice
   of potential liability as to offend the rule of lenity, the [regulation at issue here] cannot be
   one of them.”).

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           neutral interpretation of the laws Congress has enacted, defer
           to such bureaucratic pirouetting?
   Guedes, 140 S. Ct. at 790 (Gorsuch, J., statement respecting denial of certio-
   rari); see also Aposhian, 989 F.3d at 900 (“When an agency plays pinball with
   a statute’s interpretation, as the ATF has here, fair notice cannot be said to
   exist.”) (Tymkovich, C.J., dissenting); Guedes, 920 F.3d at 181 (“The ATF’s
   interpretation of ‘machinegun’ gives anything but fair warning—instead, it
   does a volte-face of its almost eleven years’ treatment of a non-mechanical
   bump stock as not constituting a ‘machinegun.’”) (Henderson, J., concur-
   ring in part and dissenting in part). 12
           The concern respecting the consistency of agency regulations is noth-
   ing new. Indeed, the Supreme Court has long recognized that an agency in-
   terpretation that “conflicts with the agency’s earlier interpretation is entitled
   to considerably less deference than a consistently held agency view.” INS v.
   Cardoza-Fonseca, 480 U.S. 421, 446 n.30 (1987) (internal quotations omit-
   ted); see also Cargill, 502 F. Supp. 3d at 1189 (quoting Cardoza-Fonseza). The
   concern is only magnified where, as here, the Government’s interpretation
   of the underlying statute carries implications for criminal liability. As such,
   Chevron does not apply because the Government has construed the same stat-
   ute in two, inconsistent ways at different points in time.
                                                V
           Turning to the rule of lenity, and assuming arguendo that the relevant
   statute is ambiguous, we now consider whether that “ambiguity concerning
   the ambit of criminal statutes should be resolved in favor of lenity.” Rewis v.

           12
              See also Thomas Z. Horton, Lenity Before Kisor: Due Process, Agency Deference,
   and the Interpretation of Ambiguous Penal Regulations, 54 Colum. J.L. & Soc. Probs. 629, 647–
   49 (2021).

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   United States, 401 U.S. 808, 812 (1971) (citation omitted). We conclude that
   the rule of lenity applies if the statute is ambiguous.
          We recognize that courts have considered two standards for whether
   a statute is sufficiently ambiguous to trigger the rule of lenity. One standard
   asks whether there is a “reasonable doubt” as to the statute’s meaning. See
   Reading Law at 299 (quoting Moskal v. United States, 498 U.S. 103, 108
   (1990)). The other inquires whether there is a “grievous ambiguity” in the
   statute. See, e.g., Chapman v. United States, 500 U.S. 453, 463 (1991) (quoting
   Huddleston v. United States, 415 U.S. 814, 831 (1974)). The Supreme Court
   does not appear to have decided which of these standards governs the rule of
   lenity. See Wooden v. United States, 142 S. Ct. 1063, 1075 (2022) (Kavanaugh,
   J., concurring) (arguing in favor of the grievous-ambiguity standard); id. at
   142 S. Ct. at 1084 (Gorsuch, J., concurring in the judgment) (arguing in favor
   of the reasonable-doubt standard).
          But it does not matter which standard applies because the rule of lenity
   applies even under the more stringent “grievously ambiguous” condition.
   One formulation of that standard provides that lenity applies if a court cannot
   discern the statute’s meaning even “after seizing everything from which aid
   can be derived.” Wooden, 142 S. Ct. at 1075 (Kavanaugh, J., concurring)
   (quoting Ocasio v. United States, 578 U.S. 282, 295 n.8 (2016)). Assuming
   that the statute at issue here is ambiguous, we can only “guess” at its defini-
   tive meaning. Suchowolski, 838 F.3d at 534. We have availed ourselves of all
   traditional tools of statutory construction, and in this circumstance, they fail
   to provide meaningful guidance. That is sufficient to require application of
   the rule of lenity irrespective of whether the reasonable doubt or grievous
   ambiguity standard applies.
          The dissenting opinion objects to our application of lenity, arguing
   that we fail to explain why the statute at issue here is grievously ambiguous.

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   With this understanding, it concludes that our holding implies that “ambig-
   uous statutes are always grievously ambiguous.” Post at 57. This criticism
   misunderstands our holding. We do not conclude that all ambiguous statutes
   are grievously ambiguous—only that this one is.
          Our conclusion fits comfortably into the dissenting opinion’s own
   conceptualization of the grievous-ambiguity standard. At the very least, len-
   ity is appropriate, the dissenting opinion concedes, if after “having tried to
   make sense of a statute using every other tool, we face an unbreakable tie be-
   tween different interpretations.” Post at 56. That is the case here in two
   respects. First, the parties argue whether “a single function of the trigger”
   refers to the firearm’s mechanics or to the shooter’s pulling of the trigger.
   Second, the parties argue if the process of engaging the trigger and maintain-
   ing forward pressure on the gun’s forebody produces “automatic” fire.
          True, the precise meaning of “grievously ambiguous” is not entirely
   clear. See Shular v. United States, 140 S. Ct. 779, 788 (2020) (Kavanaugh, J.,
   concurring) (noting that “the Court has not always been perfectly consistent
   in its formulations [of grievous ambiguity]); Reading Law at 299 (arguing that
   the term grievous ambiguity “provides little more than atmospherics, since
   it leaves open the crucial question—almost invariably present—of how much
   ambiguousness constitutes an ambiguity”) (quoting United States v. Hansen,
   772 F.2d 940, 948 (D.C. Cir. 1985)). But having utilized all available tools of
   statutory interpretation, and assuming arguendo that those two provisions are
   indeed ambiguous, we are unable to resolve either of the ties. See Alkazahg,
   81 M.J. at 784 (expressing “genuine confusion as to what the statute
   means”). That is sufficient to conclude that this statute—and for purposes
   of this case, only this statute—is grievously ambiguous.
          The dissenting opinion also objects that applying lenity in this case
   wrongfully takes Congress’s prerogative to establish federal crimes and

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   transfers that power to the Judiciary, violating the of the separation of pow-
   ers. But this is really just a repetition of the dissenting opinion’s disagree-
   ment regarding the correct interpretation of the statute at issue here. As we
   understand it, the National Firearms Act and Gun Control Act do not unam-
   biguously criminalize the possession of a non-mechanical bump stock. To
   apply lenity in this case preserves the separation of powers “by maintaining
   the legislature as the creator of crimes.” Esquivel-Quintana, 810 F.3d at 1019.
   If ATF could change the scope of criminal liability by issuing a regulation—
   free from the taxing obligations of bicameralism and presentment—the Ex-
   ecutive could wield power that our Constitution reserves to the Legislature.
          The rule of lenity also prevents the possibility whereby Congress
   passes an ambiguous criminal statute, only to be interpreted later by a federal
   agency. See id. (“By applying lenity in this setting, last of all, courts would
   avoid incentivizing Congress to enact hybrid statutes that duck under lenity’s
   imperatives, to say nothing of other imperatives in construing criminal
   laws.”). To be sure, it would be inconsistent with our constitutional struc-
   ture to apply lenity in an unprincipled manner to statutes that are not really
   ambiguous. But here, we are wholly persuaded that if the definition of “ma-
   chinegun” does not unambiguously exclude non-mechanical bump stocks, its
   inclusion of the latter is at the very least ambiguous. Given that conclusion,
   our separation of powers is aided, rather than impeded, by applying the rule
   of lenity.
          The rule of lenity is a “time-honored interpretive guideline.” Liparota
   v. United States, 471 U.S. 419, 429 (1985). We have applied it many times to
   construe ambiguous statutes against imposing criminal liability. See e.g.,
   United States v. Cooper, 38 F.4th 428, 434 (5th Cir. 2022); Kaluza, 780 F.3d
   at 669; United State v. Orellano, 405 F.3d 360, 370 (5th Cir. 2005). This case
   is no different: assuming the definition of machinegun is ambiguous, we are
   bound to apply the rule of lenity. That is, we are bound to construe the

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   definition of machinegun to exclude a semi-automatic weapon equipped with
   a non-mechanical bump stock. See Alkazahg, 81 M.J. at 784 (“We decline to
   step into the role of the legislature when the legislature has not been clear
   about whether Appellant’s conduct was criminal. Judge Henry Friendly de-
   scribed the rule of lenity as ‘the instinctive distaste against men languishing
   in prison unless the lawmaker has clearly said they should.’ Here, we express
   that distaste.”) (quoting United States v. Bass, 404 U.S. 336, 348 (1971)).
   Therefore, assuming arguendo that the statute is ambiguous, we conclude
   that the rule of lenity demands that we resolve that ambiguity in favor of Car-
   gill, and in turn conclude that a non-mechanical bump stock is not a ma-
   chinegun for purposes of the National Firearms Act and Gun Control Act.
                                         VI
          Cargill also argues that the passage of the Final Rule is an exercise of
   legislative power, in violation of the Constitution’s vesting all such power in
   Congress. U.S. Const. art. I, § 1. Some have expressed serious concern at
   the ATF’s lack of explicit authorization to interpret criminal statutes as such.
          [W]e should feel deep discomfort at allowing an agency to de-
          fine the very criminal rules it will enforce by implicit delega-
          tion. Such a delegation “turn[s] the normal construction of
          criminal statutes upside down, replacing the doctrine of lenity
          with a doctrine of severity.” Carter v. Welles-Bowen Realty, Inc.,
          736 F.3d 722, 730 (6th Cir. 2013) (Sutton, J., concurring). The
          delegation raises serious constitutional concerns by making
          ATF the expositor, executor, and interpreter of criminal laws.

   Aposhian, 989 F.3d at 900 (Tymkovich, C.J., dissenting).
          We acknowledge this concern, especially in light of statements made
   by several members the Supreme Court calling into question the relevant
   standards for legislative-power-delegation issues. See Gundy v. United States,
   139 S. Ct. 2116, 2131–42 (2019) (Gorsuch, J., dissenting, joined by Roberts,

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   C.J., and Thomas, J.); id. at 2130–31 (Alito, J., concurring in the judgment);
   Paul v. United States, 140 S. Ct. 342 (2019) (Kavanaugh, J., statement re-
   specting the denial of certiorari).
          We need not decide this question because multiple independent rea-
   sons compel us to hold the Final Rule to be unlawful. But if more were
   needed, this issue may well implicate the canon of constitutional avoidance.
   Under that well-established doctrine, “where an otherwise acceptable con-
   struction of a statute would raise serious constitutional problems, the Court
   will construe the statute to avoid such problems unless such construction is
   plainly contrary to the intent of Congress.” Hersh v. United States, 553 F.3d
   743, 754–55 (5th Cir. 2008) (quoting Edward J. DeBartolo Corp. v. Florida
   Gulf Coast Building & Constr. Trades Council, 485 U.S. 568, 575 (1988)); see
   also Reading Law at 250.
          For many jurists, the question of Congress’s delegating legislative
   power to the Executive in the context of criminal statutes raises serious con-
   stitutional concerns. See Guedes, 140 S. Ct. at 790 (Gorsuch, J., statement
   respecting denial of certiorari); Aposhian, 989 F.3d at 900 (Tymkovich, C.J.,
   dissenting); Esquivel-Quintana, 810 F.3d at 1027 (Sutton, J., concurring in
   part and dissenting in part). We do not reach this issue because we do not
   have to. But if we did, it would only provide more support for the conclusion
   that a semi-automatic rifle equipped with a non-mechanical bump stock is not
   a machinegun for purposes of federal law.
                                         VII
          Having determined that the judgment of the district court must be re-
   versed, we remand this case to the district court to enter judgment for Cargill
   and to determine the proper scope of relief. It is well-established that “[a]
   plaintiff’s remedy must be tailored to redress the plaintiff’s particular in-
   jury.” Gill v. Whitford, 138 S. Ct. 1916, 1934 (2018). And as an initial matter,

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   vacatur of an agency action is the default rule in this Circuit. See, e.g., Data
   Mktg. Partnership, LP v. United States Dept. of Labor, 45 F.4th 846, 859 (5th
   Cir. 2022) (“The default rule is that vacatur is the appropriate remedy.”);
   see also Franciscan Alliance, Inc. v. Becerra, 47 F.4th 368, 374–75 (5th Cir.
   2022) (“Vacatur is the only statutorily prescribed remedy for a successful
   APA challenge to a regulation.”). But the parties have not briefed the reme-
   dial-scope question, and it may be the case that a more limited remedy is ap-
   propriate in these circumstances. We express no opinion on that question
   other than to observe that the district court is well-placed to answer the ques-
   tion in the first instance. We therefore remand this case to the district court
   with the instruction that it enter judgment for Cargill and determine what
   remedy—injunctive, declarative, or otherwise—is appropriate to effectuate
   that judgment.
                                              VIII
           Many commentators argue that non-mechanical bump stocks contrib-
   ute to firearm deaths and that the Final Rule is good public policy. We ex-
   press no opinion on those arguments because it is not our job to determine
   our nation’s public policy. That solemn responsibility lies with the Congress,
   and our task is confined to deciding cases and controversies, which requires
   us to apply the law as Congress has written it. 13
           In defining the term machinegun, Congress referred to the mechanism
   by which the gun’s trigger causes bullets to be fired. Policy judgments aside,

           13
               The dissenting opinion accuses us of using the rule of lenity to “legalize an in-
   strument of mass murder.” Post at 61. But it is Congress’s responsibility to unambiguously
   define the scope of criminal conduct. Congress having failed to do so, we deploy lenity to
   retain the proper allocation of legislative power, not unsettle it. And the dissenting opin-
   ion’s resort to policy argument only underscores the Judiciary’s proper role. It is our re-
   sponsibility to apply the law as written, regardless of what we think about the law’s wisdom
   or utility.

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   we are bound to apply that mechanical definition. And applying that defini-
   tion to a semi-automatic rifle equipped with a non-mechanical bump stock,
   we conclude that such a weapon is not a machinegun for purposes of the Gun
   Control Act and National Firearms Act. Chevron deference likely has no role
   here either because the Government waived it or because it does not apply to
   the Government’s interpretation of a statute imposing criminal penalties. Fi-
   nally, even if the statute were ambiguous—which it is not—the rule of lenity
   would require that we interpret the statute in Cargill’s favor. As Justice
   Holmes framed it years ago, “it is reasonable that a fair warning should be
   given to the world in language that the common world will understand, of
   what the law intends to do if a certain line is passed.” McBoyle, 283 U.S. at
   27. We cannot say that the National Firearms Act and Gun Control Act give
   that fair warning that possession of a non-mechanical bump stock is a crime.
          The Final Rule promulgated by the ATF violates the APA. We there-
   fore REVERSE the judgment of the district court and REMAND with in-
   structions to enter judgment for Cargill.

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   Haynes, Circuit Judge, joined by Richman, Chief Judge, concurring in
   the judgment:
          I concur in the judgment only because I reluctantly conclude that the
   relevant statute is ambiguous such that the rule of lenity favors the citizen in
   this case.

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   James C. Ho, Circuit Judge, joined by Richman, Chief Judge, and
   Southwick, Circuit Judge, concurring in part and concurring in the
   judgment:
          Under the rule of lenity, “[p]enal statutes must be construed
   strictly.” 1 William Blackstone, Commentaries *88. “This is a
   rule of construction . . . as old and well established as law itself.” United
   States v. Wilson, 28 F. Cas. 699, 709 (C.C.E.D. Pa. 1830).
          Our courts have thus long recognized that “all reasonable doubts
   concerning [the] meaning [of a penal statute] . . . operate in favor of [the
   defendant].” Harrison v. Vose, 50 U.S. 372, 378 (1850). We apply the rule of
   lenity where “reasonable doubt persists about a [criminal] statute’s intended
   scope.” Moskal v. United States, 498 U.S. 103, 108 (1990). When standard
   principles of statutory interpretation “fail to establish that the Government’s
   position is unambiguously correct . . . [we] resolve the ambiguity in [the
   defendant’s] favor.” United States v. Granderson, 511 U.S. 39, 54 (1994).
          The rule of lenity rests on “the principle that the power of punishment
   is vested in the legislative, not in the judicial department.” United States v.
   Wiltberger, 18 U.S. 76, 85 (1820) (Marshall, C.J.). The rule also ensures fair
   notice to citizens: “To make the warning fair, . . . the line should be clear.”
   McBoyle v. United States, 283 U.S. 25, 27 (1931) (Holmes, J.).
          In sum, it is not enough to conclude that a criminal statute should cover
   a particular act. The statute must clearly and unambiguously cover the act.
          Consider the disturbing problem of designer drugs. The Controlled
   Substances Act of 1970 prohibits drugs listed on certain schedules. See Thor
   v. United States, 554 F.2d 759, 762–63 (5th Cir. 1977). In response, a line of
   synthetically produced drugs was created “to mimic the pharmacological
   effects” of scheduled drugs, while evading the Act. Clayton L. Smith, The

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   Controlled Substance Analogue Enforcement Act of 1986: The Compromising of
   Criminalization, 16 Am. J. Crim. L. 107, 108 (1988).
          Designer drugs differed in chemical composition from scheduled
   drugs. But they were just as lethal. So the same policy justifications for
   banning scheduled drugs readily applied to designer drugs.
          Yet all three branches agreed that existing law did not ban designer
   drugs. See, e.g., Controlled Substance Analogs Enforcement Act of 1985: Hearing
   on S. 1437 Before the S. Comm. on the Judiciary, 99th Cong. 1st Sess. 2 (1985)
   (opening statement of Chairman Strom Thurmond) (“[U]nlawful activity
   under the Controlled Substances Act is defined with regard to the precise
   chemical makeup of the substances described by schedules . . . . Make a minor
   alteration in the molecular structure of an outlawed drug . . . and you have
   produced a . . . dangerous narcotic that is not illegal.”); id. at 41–42
   (statement of Stephen S. Trott, Assistant Attorney General, Criminal
   Division, Department of Justice) (“[I]f a particular substance is not included
   in one of the schedules, it is not unlawful to manufacture or to distribute it . .
   . despite the potential for abuse . . . .”); United States v. Gavrilovic, 551 F.2d
   1099, 1106 (8th Cir. 1977) (reversing a conviction where a concededly
   dangerous narcotic had not been properly added to a schedule).
          So a new act of Congress was required to get at the problem of
   designer drugs. And that’s why Congress enacted the Controlled Substance
   Analogue Enforcement Act of 1986. Pub. L. No. 99-570, §§ 1201–04, 100
   Stat. 3207, 3207–13 to 3207–14 (1986). See also United States v. Muhammad,
   14 F.4th 352, 355 (5th Cir. 2021) (“[T]he Analogue Act is an antidote to
   statutory evasion: It expands the CSA’s coverage to include substances that,
   while technically not on the schedules, mimic those that are.”).
          Bump stocks present the same basic conundrum as designer drugs.
   Federal law criminalizes the possession of fully automatic machineguns. See

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   18 U.S.C. § 922(o)(1) (machinegun ban); 26 U.S.C. § 5845(b) (machinegun
   definition). That prohibition does not apply to semiautomatic weapons. See
   Gun Control Act of 1968, § 201, 82 Stat. 1231 (codified at 26 U.S.C. §
   5845(b)) (amending the 1934 federal definition of “machinegun” to omit
   weapons that shoot “semiautomatically”).
          But bump stocks now allow semiautomatic weapons to mimic
   automatic machineguns. By attaching a bump stock to a semiautomatic
   firearm, the shooter can simulate the experience of firing an automatic
   machinegun. See 83 Fed. Reg. 66,515–16.
          Just as designer drugs achieve the same lethality as scheduled drugs,
   bump stocks allow semiautomatic weapons to achieve the same lethality as
   fully automatic machineguns. But once again, Congress must take action if it
   wishes to criminalize bump stocks, just as it did during the 1980s when it
   came to designer drugs.
          That’s because federal law defines “machinegun” as a weapon that
   shoots more than one shot “automatically . . . by a single function of the
   trigger.” 26 U.S.C. § 5845(b). There are competing theories as to whether
   this language is best construed to cover bump stocks. In my own view, the
   relevant language is at best ambiguous. That makes this an easy case for
   invoking the rule of lenity. Accordingly, I agree that we should reverse.
                                         I.
          Federal law defines a machinegun as a “weapon which shoots . . .
   automatically more than one shot . . . by a single function of the trigger.” 26
   U.S.C. § 5845(b). The definition also includes “any part designed and
   intended . . . for use in converting a weapon into a machinegun.” Id.
   Violators of the machinegun ban risk up to ten years in federal prison. 18
   U.S.C. § 924(a)(2).

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          I see at least two challenges with reading § 5845(b) to cover bump
   stocks. To begin with, it’s at best ambiguous whether a semiautomatic
   weapon equipped with a bump stock is indeed capable of shooting more than
   one shot by “a single function of the trigger.” Id. And even if I could get
   past that problem, it’s also ambiguous at best whether the weapon does so
   “automatically.” Id. So the rule of lenity requires us to reverse.
                                          A.
          The phrase “single function of the trigger” is not a matter of common
   parlance. No one has identified an example of this phrase ever being used in
   any context other than this statute.
          So it’s a term that requires interpretation. And in this context, the
   interpretive steps it takes to get from “single function of the trigger” to the
   criminalization of bump stocks fall short of the fair notice that lenity requires.
          What does “single function of the trigger” mean? “[F]unction”
   means “action.” Webster’s New International Dictionary
   1019 (2nd ed. 1934). But where does that leave us?
          If it means that there is a single action on the trigger from the shooter’s
   perspective, then this language might very well apply to bump stocks. That’s
   because a single action on the trigger by the shooter is enough to spray
   multiple bullets.
          But if the phrase means that there is a single action by the trigger from
   the weapon’s perspective, then this language would not apply to bump stocks.
   That’s because from the weapon’s perspective, a single action by the trigger
   releases just one bullet. See, e.g., Gun Owners of Am., Inc. v. Garland, 992
   F.3d 446, 469 (6th Cir. 2021) (“the question is whether ‘function’ is
   referring to the mechanical process (i.e., the act of the trigger’s being
   depressed, released, and reset) or the human process (i.e., the shooter’s

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   pulling, or otherwise acting upon, the trigger)”), vacated on reh’g en banc, 2
   F.4th 576 (6th Cir. 2021).
          I conclude that grammar and syntax are at best inconclusive, and that
   lenity therefore requires us to side with the citizen over the government.
          The problem here is the distinction between what classical
   grammarians call the “subjective genitive” and the “objective genitive.”
   See, e.g., Laurel J. Brinton, The Structure of Modern
   English: A Linguistic Introduction 108 (2000) (“The phrase
   the shooting of the hunters is ambiguous between subjective and objective
   genitive readings because it can mean either ‘the hunters shoot X’ or ‘X
   shoots the hunters.’”); see also Thomas Kerchever Arnold, An
   English Grammar for Classical Schools 98 (1848); Sidney
   Greenbaum & Randolph Quirk, A Student’s Grammar of
   the English Language 103 (1990).
          Take, for example, the phrase “the love of my children.” That phrase
   could mean that my children love me (in other words, my children are the
   subject of love). Or it could mean that I love my children (so my children are
   the object of love).
          Only context can clarify whether children are the subject or the object
   of love. Compare Emma D.E.N. Southworth, Victor’s Triumph
   97 (1874) (“[S]he could not win the love of children.”) (subjective genitive),
   with Henry James, Stephen Dewhurst’s Autobiography 54 The Atlantic
   Monthly 649, 650 (1884) (“[S]he had a most vivacious love of children.”)
   (objective genitive).
          Consider another example: “the fear of the soldiers.” Without
   clarifying context, this could mean that the soldiers are fearful (the soldiers
   are the subject of fear). Or it could mean that someone else is fearful of the
   soldiers (the soldiers are the object of fear). Compare Jan Palmper Fear:

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   Soldiers and Emotion in Early Twentieth-Century Russian Military Psychology,
   68 Slavic Rev. 259, 270 n.38 (2009) (“[T]he fear that officers inspired in
   soldiers could also cause harm[] . . . . Only just, law-abiding behavior on the
   part of officers could keep this particular fear of soldiers in check.”)
   (subjective genitive), with C. Stanley Smith, Five Days, 1927 The
   Atlantic Monthly 836, 841 (“The fact that I also felt absolutely no
   fear of the soldiers contributed largely to my safety.”) (objective genitive).
          Naturally, then, this same ambiguity can also exist when a genitive
   construction modifies the term “function.”
          For example, the phrase “function of the polls” could refer to what
   polls do (polls are the subject). See A. Stuart, Norman L. Webb & D. Butler,
   Public Opinion Polls, 142 J. Royal Stat. Soc’y 443, 447 (1979) (“The
   second and most obvious function of polls is that they show the progress of
   electoral campaigns in a purely informative sense.”) (subjective genitive).
          Or it could refer to what voters do at the polls (polls are the object).
   See H.W. Warner, The Republic: Things as They Are at Present, Compared with
   the Past, 4 Am. Rev. 278, 279 (1849) (early American lawmakers established
   age, residency, and property requirements as “necessary qualifications for
   the function of the polls”) (objective genitive).
          By the same token, a “function of the trigger” could mean what the
   trigger does (the trigger is the subject). Or it could mean what the shooter
   does to the trigger (the trigger is the object). A “single function of the
   trigger” could mean that the trigger acts once—or that the shooter acts once
   on the trigger.
          The government’s reading prevails if it’s clear that the trigger is the
   object, and the shooter is the implied subject. But there’s nothing inherent
   about the phrase “function of the trigger” that tells us that the trigger is the
   object rather than the subject.

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           So grammar alone can’t dictate whether we should read the statute
   from the machine’s perspective (subjective genitive) or the shooter’s
   perspective (objective genitive). As the D.C. Circuit explained: “[T]he text
   is silent on the crucial question of which perspective is relevant.                       A
   mechanical perspective . . . might focus on the trigger’s release of the
   hammer, which causes the release of a round. From that perspective, a
   ‘single function of the trigger’ yields a single round of fire when a bump-stock
   device moves the trigger back and forth. By contrast, from the perspective of
   the shooter’s action, the function of pulling the trigger a single time results
   in repeated shots when a bump-stock device is engaged.                         From that
   perspective, then, a ‘single function of the trigger’ yields multiple rounds of
   fire.” Guedes v. Bureau of Alcohol, Tobacco, Firearms & Explosives, 920 F.3d 1,
   29 (D.C. Cir. 2019).
           In the examples provided above, the history or context of a particular
   phrase sheds light on whether it contains a subjective or objective genitive.
   Take another example: “The fear of the Lord is the beginning of wisdom.”
   Psalm 111:10. Construed as a subjective genitive, “the fear of the lord”
   could refer to an anxious aristocrat, afraid of an overweening monarch or
   unruly populace.        But we know that the Bible means something very
   different—that the wise man fears God.
           Unlike “fear of the Lord,” a “single function of the trigger” does not
   offer much by way of history or context. No example of its usage in any other
   context has been provided by anyone in this litigation. Congress appears to
   have coined the phrase just for this statute. 1

           1
            The plurality claims that its interpretation is supported by context: A neighboring
   provision, § 5845(c), refers to a “single pull of the trigger”—whereas § 5845(b) uses the
   phrase “single function of the trigger.” See ante, at 20–21. So the plurality infers that the

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           As between construing “single function of the trigger” from the
   weapon’s perspective or the shooter’s perspective, then, the statute appears
   to be in equipoise. And statutory equipoise is a textbook case for lenity. 2

   term “pull” must be distinct from the term “function”—if “pull” signifies the shooter’s
   perspective, then “function” must signify the weapon’s perspective. Cf. Gun Owners, 992
   F.3d at 469 (“pull” suggests “shooter’s” perspective); Guedes, 920 F.3d at 29 (same).
            But the dissent would presumably counter with legislative history suggesting that
   the terms are interchangeable rather than distinct. See Cargill v. Garland, 20 F.4th 1004,
   1010 (5th Cir. 2021), reh’g granted, 37 F.4th 1091 (5th Cir. 2022) (quoting House report
   statement that the statute defines machinegun as “a weapon designed to shoot more than
   one shot without reloading and by a single pull of the trigger”); id. (statement of Karl T.
   Frederick, President, National Rifle Association of America) (equating “single pull of the
   trigger” with “single function of the trigger”). And whatever one may think of legislative
   history, the Supreme Court has repeatedly looked to it before invoking lenity. See, e.g.,
   Moskal, 498 U.S. at 108; Granderson, 511 U.S. at 54.
           So there’s some evidence that the terms are distinct, and some that the terms are
   interchangeable. The evidence thus appears to be in equipoise—a classic case for lenity.
           2
             Both the plurality and the dissent resist the notion of equipoise. They both point
   to other words in § 5845(b) that arguably favor their respective positions: The plurality
   invokes the subject of the sentence (“machinegun”)—while the dissent relies on a prepo-
   sitional phrase (“without manual reloading”).
            Neither inference seems warranted. Neither the plurality nor the dissent cites a
   rule of grammar that says that a subject or prepositional phrase tells us whether to read a
   subsequent phrase as a subjective or objective genitive. And I’m aware of none.
            For example, the plurality notes that, under § 5845(b), “machinegun means . . .
   any weapon which shoots . . . automatically more than one shot . . . by a single function of
   the trigger.” 26 U.S.C. § 5845(b). So “[t]he subject of the sentence . . . is machinegun.”
   Ante, at 19. Therefore, the plurality argues, it must be the perspective of the machinegun,
   not the shooter, that dictates how we read “function of the trigger.”
           But no rule of grammar supports this inference. Suppose I offer this definition:
   “Sugar means a sweet carbohydrate that enhances the enjoyment of food.” Naturally, we
   would read “enjoyment of food” from the perspective of the eater, not the food—even
   though sugar is the subject of the sentence, and the sentence never mentions the eater.
           The dissent commits a similar error. It claims that § 5845(b) must be read from
   the shooter’s perspective because it defines “machinegun” to mean a weapon that fires

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                                               B.
           There’s another ambiguity in the statute. Does a semiautomatic
   weapon equipped with a bump stock shoot multiple bullets “automatically?”
   Each side puts forth its competing theory with great force. But neither deals
   a fatal blow to the other—which is why, once again, lenity compels reversal.
           Cargill argues that a bump stock doesn’t shoot multiple bullets
   “automatically” because it requires “constant forward pressure with the
   non-trigger hand.” Guedes, 920 F.3d at 44 (Henderson, J., concurring in part
   and dissenting in part). “Automatic” means “self-acting.” Oxford
   English Dictionary at 574 (1933). So Cargill argues that the need for
   human input means that bump stocks do not shoot automatically, because
   they do not shoot in a self-acting fashion.
           But the government counters that the term “automatic” need not
   necessarily mean no human input. It could just mean less human input. And
   that would make it ambiguous at best whether “automatic” includes or
   excludes bump stocks.
           Consider an automatic sewing machine. With a machine that sews
   automatically, you don’t just push a button—you also move the cloth forward
   with your hand.

   multiple shots, “without manual reloading,” by a single function of the trigger. 26 U.S.C.
   § 5845(b). The phrase “without manual reloading” means that no reloading is performed
   by the shooter. So the dissent theorizes that “single function of the trigger” must likewise
   refer to an action performed by the shooter. See post, at 57 n.5.
            But once again, no rule of grammar supports such an inference. Suppose I offer
   this definition: “A well-trained army means an army that follows orders, without constant
   reminding, due to the fear of the soldiers.” Naturally, “without constant reminding” means
   no reminding is done by officers, not soldiers. Yet the phrase “fear of the soldiers” obvi-
   ously refers to a feeling felt by soldiers. The soldiers are afraid—not the officers.

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          This is similar to a bump stock. You don’t just pull the trigger with
   your finger—you also apply pressure with your non-trigger hand. See Guedes,
   920 F.3d at 30.
          Likewise,    with     an   automatic      car—one    that   shifts   gears
   automatically—you still have to “maintain[] enough constant pressure on
   the gas pedal to reach a speed that triggers a gear shift.” Gun Owners of Am.,
   Inc. v. Garland, 19 F.4th 890, 906 (6th Cir. 2021) (en banc) (White, J., for an
   equally divided court).
          “[T]he ultimate question,” then, “is how much human input is
   contemplated by the word ‘automatically.’” Id. But “[t]hat is a question of
   degree that the statute’s text does not definitively answer.” Id. And because
   there is no definitive answer, lenity compels reversal.
                                         II.
          The rule of lenity “requires ambiguous criminal laws to be interpreted
   in favor of the defendants subjected to them.” United States v. Bittner, 19
   F.4th 734, 748 (5th Cir. 2021) (quoting United States v. Santos, 553 U.S. 507,
   514 (2008) (plurality opinion)), cert. granted, 142 S. Ct. 2833 (2022). Lenity
   applies when there “remains a grievous ambiguity or uncertainty,” even after
   the court has examined it using the standard tools of construction. United
   States v. Castleman, 572 U.S. 157, 173 (2014).
          Bump stocks may well be indistinguishable from automatic weapons
   for all practical purposes. But as Chief Justice Marshall recognized two
   centuries ago in a seminal case on the rule of lenity: “It would be dangerous
   . . . to punish a crime not enumerated in the statute, because it is of equal
   atrocity, or of kindred character, with those which are enumerated.”
   Wiltberger, 18 U.S. at 96.

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           Consider the facts in Wiltberger. A shipmaster had killed a sailor while
   the vessel was on a river, near its mouth. Id. at 77. Surely we would all agree
   that manslaughter is a criminal act, deserving of punishment. The Court
   nevertheless unanimously construed a statute that punished manslaughter on
   the “high seas” not to apply to an identical act on a river. Id. at 103–06. The
   Court noted that it was “extremely improbable” Congress would want to
   treat upstream manslaughter differently from manslaughter committed
   downstream, past the river’s mouth. Id. at 105. “But probability is not a
   guide which a court, in construing a penal statute, can safely take.” Id. 3

           3
           The dissent says lenity applies only in cases of true equipoise—where there’s an
   “unbreakable tie” between competing interpretations. Post, at 56. See also id. at 58.
            But equipoise is precisely what’s presented here. As explained, each side offers
   conflicting theories as to whether bump stocks fire multiple bullets “automatically” or “by
   a single function of the trigger.” But neither has the goods on the other. So lenity governs.
           Moreover, the Supreme Court has long held that lenity requires us to “resolve []
   ambiguity” and construe “reasonable doubt” in favor of the accused. See Granderson, 511
   U.S. at 54; Moskal, 498 U.S. at 108. The dissent responds that more recent precedent in-
   vokes lenity only in cases of “grievous ambiguity.” Post, at 56 & n.2. But the Court has
   never indicated any intention to abrogate its longstanding commitment to lenity in cases of
   “reasonable doubt.”
             And for good reason: The Court has historically “link[ed] the high burden of the
   rule of lenity with the high burden of proving guilt in a criminal trial beyond a reasonable
   doubt.” Daniel Ortner, The Merciful Corpus: The Rule of Lenity, Ambiguity and Corpus Lin-
   guistics, 25 B.U. Pub. Int. L.J. 101, 109 (2016).
              The link between legal and factual doubt is not just longstanding—it’s logical. Af-
   ter all, it’s just as “grievous” to punish an accused whether there’s reasonable doubt as to
   fact or law. See id.; see also Antonin Scalia & Bryan A. Garner, Reading
   Law: The Interpretation of Legal Texts 296, 299 (2012) (lenity reflects
   “‘the tenderness of the law for the rights of individuals,’” Wiltberger, 18 U.S. at 95, and
   applies where a matter is “not beyond reasonable doubt,” because “the consequences
   should be visited on the party more able to avoid and correct the effects of shoddy legisla-
   tive drafting”).

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                                               ***
           As a matter of lethality, bump stocks may well be no different from
   machineguns.        Just as manslaughter on a river is no less deadly than
   manslaughter at sea.          And designer drugs are no less dangerous than
   scheduled drugs. But as a matter of legality, Congress could not ban designer
   drugs without passing the Analogue Act. Nor could it punish manslaughter
   on a river without saying so. Likewise, Congress cannot criminalize bump
   stocks absent a clear and unambiguous statute.
           Members of Congress who strongly oppose bump stocks nevertheless
   concede that “legislation is the only way to ban bump stocks.” Press Release,
   Sen. Dianne Feinstein, Feinstein Statement on Regulation to Ban Bump Stocks
   (Mar. 23, 2018). I agree. 4

           4
              Thirteen of the sixteen members of our en banc court likewise agree that legisla-
   tion is the “only way” to ban bump stocks. Id.
            The dissent responds by accusing 80% of our court of “legaliz[ing] an instrument
   of mass murder.” Post, at 61. Yet the dissent does not accuse the Supreme Court of “le-
   galiz[ing] . . . murder”—even though it applied lenity to manslaughter in Wiltberger. To
   the contrary, the dissent relies on Wiltberger. See id. at 58 (quoting Wiltberger, 18 U.S. at
   95).
             Odder still, the dissent also accuses us of creating a “special rule of lenity for
   guns,” “giving machinegun owners immunity from prosecution that is not shared by other
   offenders.” Id. at 57 n.4, 61. I have no idea what “special rule” the dissent is talking about.
   I’ll say it again: I would apply the exact same principle to guns as to drugs or manslaughter.
            Finally, the dissent theorizes that, if we apply lenity here, “it is unclear how Con-
   gress could draft [] a [new] statute while avoiding ambiguity as to what counts as a bump
   stock.” Post, at 61. But what’s wrong with the text proposed by Senator Feinstein after the
   Las Vegas shooting—which makes it unlawful to possess “a bump-fire device, or any part,
   component, device, attachment, or accessory that is designed or functions to accelerate the
   rate of fire of a semiautomatic rifle but not convert the semiautomatic rifle into a ma-
   chinegun”? S. 1916, 115th Cong. § 2 (2017). The dissent does not say.

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   Stephen A. Higginson, Circuit Judge, joined by Dennis and
   Graves, Circuit Judges, dissenting:
                                          I.
          For the reasons stated in the panel opinion, Cargill v. Garland, 20
   F.4th 1004 (5th Cir. 2021), reh’g granted, 37 F.4th 1091 (5th Cir. 2022), I
   respectfully dissent from our court’s decision that a bump stock is not a
   machinegun within the meaning of 18 U.S.C. § 921(a)(24).
                                         II.
          I write further to dissent from our court’s use of lenity to rewrite this
   statute.
          The Supreme Court has repeatedly instructed that “the rule of lenity
   only applies, if, after considering text, structure, history, and purpose, there
   remains a grievous ambiguity or uncertainty in the statute such that the [c]ourt
   must simply guess as to what Congress intended.” Maracich v. Spears, 570 U.S.
   48, 76 (2013) (emphases added) (cleaned up); see, e.g., Ocasio v. United States,
   578 U.S. 282, 295 n.8 (2016) (similar); Robers v. United States, 572 U.S. 639,
   646 (2014) (similar); United States v. Hayes, 555 U.S. 415, 429 (2009)
   (similar). Under this standard, the Supreme Court has been clear that we do
   not invoke lenity just because “multiple, divergent principles of statutory
   construction” are available, Lockhart v. United States, 577 U.S. 347, 361
   (2016), “the statute’s text, taken alone, permits a narrower construction,”
   Abramski v. United States, 573 U.S. 169, 188 n.10 (2014), or “a law merely
   contains some ambiguity or is difficult to decipher,” Wooden v. United States,
   142 S. Ct. 1063, 1075 (2022) (Kavanaugh, J., concurring). Rather, the
   Supreme Court lets us deploy lenity to narrow laws only as a last resort when,

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   having tried to make sense of a statute using every other tool, we face an
   unbreakable tie between different interpretations. 1
           Contrary to this authority, the majority opinion and the lead
   concurrence apply the rule of lenity to garden-variety ambiguity. 2 In doing
   so, today’s ruling usurps Congress’s power to define what conduct is subject
   to criminal sanction and creates grave ambiguity about the scope of federal
   criminal law.
           Under the majority’s rule, the defendant wins by default whenever the
   government fails to prove that a statute unambiguously criminalizes the
   defendant’s conduct.             The majority holds that § 921(a)(24) is
   “unambiguous,” but claims that if the statute were ambiguous, it would

           1
             Notwithstanding this Supreme Court precedent, there is robust scholarly debate
   about how much ambiguity triggers lenity. See, e.g., David S. Romantz, Reconstructing the
   Rule of Lenity, 40 Cardozo L. Rev. 523, 567 (2018) (cataloguing nine tests); Intisar S.
   Rabb, The Appellate Rule of Lenity, 131 Harv. L. Rev. F. 179 (2018) (conducting empir-
   ical study of lenity cases); Shon Hopwood, Restoring the Historical Rule of Lenity as a Canon,
   95 N.Y.U. L. Rev. 918 (2020) (attacking the modern approach). This debate has crossed
   over to sitting Supreme Court Justices, who are free to explore whether they might change
   the law that binds us. Compare Shular v. United States, 140 S. Ct. 779, 788 (2020) (Ka-
   vanaugh, J., concurring), and Wooden, 142 S. Ct. at 1075 (Kavanaugh, J., concurring), with
   Wooden, 142 S. Ct. at 1084 (Gorsuch, J., concurring in the judgment) (questioning the
   “grievous” ambiguity standard).
           2
              In this respect, today’s ruling departs from our many cases that follow binding
   Supreme Court law. See, e.g., Gonzalez v. CoreCivic, Inc., 986 F.3d 536, 539 (5th Cir. 2021)
   (Ho, J.) (affirming that the rule of lenity “has force only where a law is grievously ambigu-
   ous, meaning that the court can make no more than a guess as to what the statute means”
   (cleaned up)). However, the recent trend in our circuit, culminating here, has been to lower
   the bar for lenity beneath the floor presently set by the Supreme Court. See, e.g., United
   States v. Hamilton, 46 F.4th 389, 397 n.2 (5th Cir. 2022) (Elrod, J.) (applying lenity to “re-
   solve all reasonable doubts about the meaning of [the criminal statute] in [the defendant’s]
   favor” and asserting that lenity applies where “there is some doubt about the meaning” of
   the statute).

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   invoke the rule of lenity. 3 In making this assertion, the majority assumes that
   the statute would necessarily be so ambiguous that “all traditional tools of
   statutory construction” would “fail to provide meaningful guidance.” Yet
   the majority does not explain how the tools upon which it relied to interpret
   the statute—dictionaries, grammar, and corpus linguistics—would be
   useless to resolve an interpretive debate if the statute were ambiguous. So
   the majority rests on an unstated and unsupported leap: ambiguous statutes
   are always grievously ambiguous. In effect, this means the rule of lenity
   would apply to decide any ambiguity in Cargill’s favor. 4
           The lead concurrence adopts an equally low threshold for lenity.
   Unlike the majority, the concurrence concludes that § 921(a)(24) is
   ambiguous. But instead of relying on familiar techniques to resolve the
   ambiguity, the concurrence merely asserts that this is “an easy case for
   invoking the rule of lenity.” The concurrence first invokes lenity because it
   cannot decide whether “single function of the trigger” means that “the
   trigger acts once” or “the shooter acts once on the trigger,” and so “the
   statute appears to be in equipoise.” This dilemma is of the concurrence’s
   own making. The concurrence contrives an impossible task by isolating the
   phrase “single function of the trigger” from the rest of the provision. 5

           3
             The only other court to find that bump stocks are not machineguns made a similar
   mistake. See United States v. Alkazahg, 81 M.J. 764, 784 (N-M. Ct. Crim. App. 2021) (as-
   serting that defendant would prevail thanks to lenity if the court’s “statutory analysis
   [were] incorrect and the ambiguity could not be resolved”).
           4
             The majority insists that this rule is limited to “this statute.” But by devising a
   special rule of lenity for guns, the majority substitutes its own policy preferences for Con-
   gress’s.
           5
            Notably, the concurrence ignores the phrase “without manual reloading,” which
   immediately precedes “by a single function of the trigger” and refers to the action of a
   shooter (implied subject) on a gun (object). See 26 U.S.C. § 5845(b). It does so because

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   Further, the concurrence refuses to explain why it could not weigh relevant
   evidence, including legislative history and District Judge Ezra’s inferences
   drawn from expert testimony at trial, see Cargill, 20 F.4th at 1010, 1013, to
   determine whether the “mechanistic” interpretation prevails.
           The lead concurrence next invokes lenity because the text does not
   “definitive[ly] answer” the question of whether the statutory term
   “automatically” means “no human input,” not “less human input.” But
   just because both Cargill and the government “put[] forth [their] competing
   theories with great force” does not mean we are left to guess at and then
   invalidate what Congress intended. Maracich, 570 U.S. at 76 (cleaned up).
           More fundamentally, our court’s new lenity regime violates
   separation-of-powers principles. Article I gives Congress, “not the [c]ourt,”
   the power to “define a crime, and ordain its punishment.” United States v.
   Wiltberger, 18 U.S. (5 Wheat.) 76, 95 (1820). As the Supreme Court has
   resolved, when properly limited to grievous ambiguity, lenity furthers this
   design. By breaking interpretive ties for the defendant where no other tool
   yields an answer, this canon keeps us from accidentally legislating crimes
   from the bench. But lenity is the enemy of Article I when applied to any
   ambiguity that might arise during statutory interpretation and that could be
   resolved using other interpretive tools. This is because statutory language
   can be ambiguous enough to bear multiple interpretations—even here, a

   “no rule of grammar” would compel us to read the statute “from the shooter’s perspec-
   tive.” But the grammar is clear: a shooter is the implied subject of the sentence. Even if
   this conclusion were not apparent from context, we do not decide what statutes mean by
   drawing inferences from the absence of a grammatical postulate—we use common sense.
   See United States v. Castleman, 572 U.S. 157, 183 (2014) (Scalia, J., concurring in part and
   concurring in the judgment); RadLAX Gateway Hotel, LLC v. Amalgamated Bank, 566 U.S.
   639, 645 (2012) (Scalia, J.). Here, the concurrence’s reading has the strange effect of an-
   thropomorphizing the gun and eliding the shooter from the statute.

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   “mechanistic” one—yet still evince intent to sanction specific conduct.
   Compare Hayes, 555 U.S. at 429 (acknowledging that a statute was “not a
   model of the careful drafter’s art” but declining to apply lenity where “text,
   context, purpose, and what little there is of drafting history all point in the
   same direction”), and United States v. Palomares, 52 F.4th 640, 647 (5th Cir.
   2022) (declining to apply lenity where “one approach [stood] prominently
   above the other interpretations”), with Ladner v. United States, 358 U.S. 169,
   178 (1958) (applying lenity where the choice between interpretations would
   “be based on no more than a guess as to what Congress intended”); cf. Einer
   Elhauge, Preference-Eliciting Statutory Default Rules, 102 Colum. L. Rev.
   2162, 2196 (2002) (In the criminal law context, “judicial resolution of
   statutory ambiguities does not tread on the legislative role, but rather
   executes the legislative instructions as best as judges can.”). Invoking lenity
   to avoid all ambiguity thwarts the meaning of the text and substitutes our
   judgment for the People’s about what counts as a crime. 6
            Because our court holds that we have the power to narrow federal
   criminal law where ambiguity appears, today’s ruling, which conflicts with
   how every other circuit has interpreted § 921(a)(24), calls into question the
   range of conduct subject to criminal sanction. 7                 Among other recent

           6
              Such an expansive rule of lenity also “compels judges to abdicate the judicial
   power without constitutional sanction.” Baldwin v. United States, 140 S. Ct. 690, 691
   (2020) (Thomas, J., dissenting from denial of certiorari). The Article III judicial power
   “requires a court to exercise its independent judgment in interpreting and expounding
   upon the laws” and “include[s] the power to resolve . . . ambiguities.” Perez v. Mortg.
   Bankers Ass’n, 575 U.S. 92, 119 (2015) (Thomas, J., concurring in the judgment). There-
   fore, when lenity is used as a get-out-of-interpretation-free card—here, to avoid passing on
   the legality of machineguns—it is especially inconsistent with our constitutional role.
           7
             Of course, most criminal statutes are drafted by state legislatures, responding to
   imminent and present threats to public safety. Prohibitions on dangerous weapons are myr-
   iad. Many ban machineguns and define them, unambiguously I would say, along similar
   lines as Congress did in 1934. See, e.g., Tex. Penal Code § 46.01(9).

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   decisions, we may have wrongly held that criminal defendants are ineligible
   for the First Step Act’s “safety valve” provision, 18 U.S.C. § 3553(f), if they
   fail to meet any one of the statute’s three requirements, see Palomares, 52
   F.4th at 647, and that 26 U.S.C. § 7202 criminalizes the willful failure to
   “either truthfully account for taxes or pay them over,” United States v.
   Sertich, 879 F.3d 558, 562 (5th Cir. 2018), because those statutes are not
   unambiguous. Moreover, given the ambiguities the concurrence perceives in
   “single function of the trigger” and “automatically,” it is probable that other
   machineguns cannot be outlawed under § 921(a)(24)—even guns that fire
   multiple bullets when the shooter holds down the trigger. After all, the
   concurrence says that it’s plausible that “automatically” means “no human
   input,” and the pressure needed to depress a trigger is plausibly human input
   within the meaning of the statute. 8
           The concurrence argues that Congress could pass an “Analogue Act”
   that would explicitly go device by device, mechanistically, and define bump
   stocks as machineguns. This would work, the concurrence hypothesizes,
   because Congress passed a similar statute in the controlled substances
   context to regulate chemicals that were analogues to drugs named in an
   earlier act. See 21 U.S.C. §§ 802(32)(A), 813, 841(a)(1); McFadden v. United
   States, 576 U.S. 186, 188 (2015). Setting aside the obvious differences
   between guns and drugs—including the fact that a drug can be described with

           8
             For example, devices called “auto-sears” that allow semiautomatic weapons to
   fire multiple rounds while the trigger is pulled are increasingly accessible to criminals. See
   Alain Stephens & Keegan Hamilton, The Return of the Machine Gun, The Trace (Mar.
   24, 2022). Even though these devices gravely threaten public safety and law enforcement,
   see Michelle Homer & Melissa Correa, ‘This Has to Stop’: 19 Houston-Area Suspects Charged
   in Federal Crackdown on Illegal Gun Switches, KHOU (Feb. 24, 2022); Florian Martin, The
   Man Who Shot and Killed an HPD Officer Last Week Used an Illegally Modified Handgun,
   Bodycam Footage Shows, Hous. Pub. Media (Oct. 13, 2021), it is uncertain after today’s
   ruling whether federal law can reach them.

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   a molecular formula and a gun cannot—the concurrence is mistaken. Under
   our court’s new lenity regime, it is unclear how Congress could draft such a
   statute while avoiding ambiguity as to what counts as a bump stock. As I
   explained, any ambiguity in the statute could be exploited to evade liability
   because those ambiguities must be construed in favor of defendants.
          Indeed, after our court’s ruling today, it is not clear that the
   Controlled Substance Analogue Enforcement Act of 1986 is even operable in
   practice. In relevant part, this statute defines a “controlled substance
   analogue” as a substance “the chemical structure of which is substantially
   similar to the chemical structure of a controlled substance in schedule I or
   II.” 21 U.S.C. § 802(32)(A)(i) (emphasis added). Whether two substances
   have a “substantially similar” chemical structure may, in many cases, be
   ambiguous such that lenity would shield a manufacturer, distributor, or
   possessor of the analogue. This is not the result Congress intended.
                                         III.
          Today, our court extends lenity, once a rule of last resort, to rewrite a
   vital public safety statute banning machineguns since 1934. In conflict with
   three other courts of appeals, our court employs its new lenity regime to carve
   out from federal firearms regulation the bump stock—a device that helped
   the Las Vegas shooter fire over a thousand rounds during an eleven-minute-
   long attack, at times shooting about nine bullets per second, killing at least 58
   people and wounding hundreds more. See Larry Buchanan et al., What Is a
   Bump Stock and How Does It Work?, N.Y. Times (updated Mar. 28, 2019).
   Therefore, our court uses lenity to legalize an instrument of mass murder.
   This is evident from our court’s attempt to confine its new lenity regime only
   to this statute, giving machinegun owners immunity from prosecution that is
   not shared by other offenders under the federal code.

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          For those reasons and the reasons stated in the panel opinion, I
   respectfully dissent.

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