The SIG Sauer P320 is a modular semi-automatic pistol made by SIG Sauer, Inc., SIG Sauer's American branch. It is a further development of the SIG Sauer P250, utilizing a striker-fired mechanism in lieu of a double action only hammer system. The P320 can be chambered in 9×19mm Parabellum, .357 SIG, .40 S&W, .45 ACP, and 10mm Auto and can be easily converted from one caliber to another; a change from .357 SIG to .40 S&W requires only a barrel change; a change from 9mm to .357 SIG or .40 S&W (or vice versa) is accomplished using a caliber exchange kit.
 The P320 chambered in 9×19 mm Parabellum was introduced in the North American market on 15 January 2014, followed by the .45 ACP compact model at the SHOT Show in January 2015. On 19 January 2017, it was announced that a customized version of the SIG Sauer P320 had won the United States Army's XM17 Modular Handgun System competition. The full-sized model will be known as the M17 and the compact carry-sized model will be known as the M18.
 According to The Washington Post, the P320 is one of the most popular handguns in America.
 The P320 was designed to be ambidextrous in handling, sporting a catch lever on both sides of the slide and user-reversible magazine release, and all other operating controls are designed so they can be operated from either side.  The firearm can be field stripped with no tools.  Additionally, the firearm can also be field stripped without depressing the trigger, an additional safety feature to prevent negligent discharge of the weapon. The fire control unit is removable, allowing the switch to a different frame size.
 The P320 trigger shoe is available in an all-metal, solid trigger.
 The requirements for the new US Army handgun included the idea that an existing handgun model would be preferred for the Modular Handgun System Request for Proposal, known as the XM17 Procurement. SIG Sauer submitted a P320 with a number of modifications and submitted them for the XM17 Modular Handgun System competition.
 Modifications include:
 On 19 January 2017, it was announced that the SIG Sauer P320 MHS variant had won the United States Military's Modular Handgun System trials. The P320 will be known as the M17 (full size) and M18 (compact) in U.S. Military service. Though the pistol will remain chambered in 9 × 19 mm Luger rather than a larger caliber, the contract allows the services to procure SIG Sauer's proposed XM1152 Full Metal Jacket and XM1153 Special Purpose ammunition.
 In May 2017, the Army announced that the first unit that will receive the M17 would be the 101st Airborne Division by the end of the year. At the same time, the rest of the U.S. Armed Forces revealed they also intend to acquire the handgun, making it the standard sidearm for the entire U.S. military. The services plan to procure up to 421,000 weapons in total; 195,000 for the Army, 130,000 for the Air Force, 61,000 for the Navy (XM18 compact version only), and 35,000 for the Marines.
 On 17 November 2017, soldiers of the 101st Airborne received the first XM17 and XM18 pistols, with over 2,000 handguns delivered. The XM17 has better accuracy and ergonomics and tighter dispersion than the M9. It will also be fielded more widely, being issued down to squad and fireteam leaders; while special forces would dual-arm all of its members with a pistol and rifle, previously junior leaders in regular infantry units were excluded from carrying sidearms but policy was changed to give them more choices and options in close quarters battle situations. All Army units are planned to have the M9 replaced with the M17 within a decade.
 Initial production models of the P320 were found to have a "drop safety" issue if the firearm was dropped at a specific angle, potentially causing it to discharge. SIG Sauer has since refitted the P320 to make it drop-safe and offers a voluntary upgrade program for early P320s.
 Issues of drop firing or shooting its user in the leg while holstered have continued sporadically with at least six departments removing the P320 from service. As of April, 2023, an investigation has shown more than 100 people have reported that their own P320 fired without someone pulling the trigger, wounding at least 80 of them, including 33 officers at 18 law enforcement agencies.
 Around 400 P320s were procured for the Canadian Joint Task Force 2 special forces unit (JTF-2) in 2019, but these were withdrawn and the earlier P226 pistols (also manufactured by SIG Sauer) reinstated following a misfire that injured a soldier during a training exercise in November 2020; JTF-2 was the only Canadian military unit using the P320. In June 2021, a technical investigation found that the misfire was due to "a partial depression of the trigger by a foreign object combined with simultaneous movement of the slide [...] that then allowed a round to be fired whilst the pistol was still holstered" and that the usage of a holster designed for a different pistol was a contributory factor; the P320 itself was not at fault nor were there any issues with how it had been procured by Canadian defence officials (since questions had been raised as to whether these officials were aware of the drop safety issues). However, the pistols would remain in storage pending a third-party safety assessment. The safety assessment and other proceedings related to the misfire were concluded by June 2022, with a decision to reinstate the P320 being taken towards the end of that month.
 The X Series lineup includes the following grip module sizes:
 In January 2019, SIG Sauer announced the XCompact handgun as the newest entry in their X Series lineup.
 The XCompact size grip module is the smallest grip module SIG currently carries, as they have not come out with a subcompact X Series grip module to date.
 In May 2022, SIG Sauer announced a 10mm Auto P320 variant, named the P320 XTEN.
 The X-Carry is a larger model of the XCompact featuring 17 rounds per magazine, compared to the Compacts 15 rounds, enlarged grip, longer slide and a smaller slide lock. The X-Carry is the frame used for the M18
 Released in late July/early August 2019 the XFIVE Legion is considered the flagship of the P320 platform that brings added weight and features.  The TXG grip module has tungsten infused directly into the polymer along with an attachable magazine well.  It comes standard with Henning group aluminum base pads and a skeletonized flat trigger.  The complete 9mm slide has lightening cuts in the top to reduce weight and assist in recoil.  It is also optics ready through the removal of the rear sight plate assembly. The optics cut is the Delta Point/Romeo 1 Pro profile, but will accommodate multiple optics via adapter plates.
 The SIG P320-XTENMAX is a 10mm Auto chambered X Series variant of the P320, released in 2022. It features an X Series grip module, bull barrel, flat X Series trigger and a Nitron-finished stainless steel slide with a cut for mounting aiming optics. Magazine capacity is 15 rounds. It weighs 33 oz (936 grams), has a 5-inch (127 mm) bull  barrel, overall length of 8.5 inches (216 mm), overall height of 5.6 inches (142 mm), overall width of 1.3 inches (33 mm) and a sight radius of 6.8 inches (173 mm).
 The SIG P320MAX is a sporterised variant of the P320, designed in 2021 for use by competition shooters. The pistol comes with ROMEO3 MAX sights and tungsten infused TXG heavy grip module, weighs 43.5 oz (1.23 kilograms), has a 5-inch (130 mm) match grade barrel, and an overall length of 8.5 inches (220 mm).
 In late July 2017, the Dallas Police Department in Texas instructed all personnel to stop carrying the P320 pending an investigation.  There were concerns that the firearm may discharge when it is dropped and the back of the slide hits the ground at a 33-degree angle.  The problem was thought to be related to the trigger weight; some triggers were heavy enough that they essentially continued to move due to inertia after the gun hit the ground.  Internet publications, such as TheTruthAboutGuns.com, conducted independent tests that appeared to confirm potential drop firings (at a 40 percent rate).
 On 8 August 2017, SIG Sauer issued a notice that they would upgrade all P320s to address the issue. The upgrade is described on the company's website as: "an alternate design that reduces the physical weight of the trigger, sear, and striker while additionally adding a mechanical disconnector."
 A 2023 investigation by The Washington Post and The Trace found more than 100 people who say their P320 discharged with no trigger pressure; at least 80 of them were wounded as a result. At least 35 of the shootings are purported to have occurred with either post-2017 P320s with the updated design intended to address the drop firing problem, or older ones which had been updated by Sig Sauer.
 Unlike other striker-fired guns, the P320 is "effectively fully cocked at rest", since its striker is under constant spring pressure, which is released when the trigger is pulled. Most such guns, including the P320's military variant, have external safeties (like thumb safeties), which the P320's civilian variant lacks. According to gunsmith James Tertin, this is a rare and "uniquely dangerous" configuration. Sean Toner, the P320 designer, has acknowledged in court that "U.S. standards are not very stringent." There are no U.S. (government) standards on gun safety, since blocked by law.
 By April 2023, six American law enforcement agencies[which?] discontinued use of the firearm due to concerns over unintentional discharges. The Fraternal Order of Police's Wisconsin state lodge recommended in mid-September 2022 for the state's police departments to stop using the weapon, claiming three Milwaukee Police officer injuries involving them or their partner's service weapons firing without any trigger pressure since 2020.
 Sig Sauer responded by asserting that most accidental discharges involve negligence; that many other gun models suffer from such discharges; that the P320 has been extensively tested and meets safety standards; and that, despite years of complaints and litigation, no one has successfully recreated the claimed phenomenon by demonstrating a P320 firing without its trigger being pulled.
 In May 2017, Steyr Mannlicher filed a patent infringement lawsuit against SIG Sauer. Steyr refers to their patent US6260301 (filed in 1999 and approved in 2001), which is for a handgun with a removable chassis. Steyr Arms requested a preliminary and permanent injunction against SIG Sauer selling any such firearms. On 25 February 2020, the United States District Court for the District of New Hampshire granted Steyr summary judgment, finding that SIG Sauer did not infringe Steyr's patents.
 A lawsuit related to the above-noted drop firing problem and filed in April 2018 in the United States District Court for the Western District of Missouri led to a class action settlement in February 2020. Elements of the agreement include:
 A class action settlement form is available on the SIG Sauer website.
 In September 2019, an Arizona gun owner who purchased a P320 in September 2016 initiated a class action lawsuit. It claims that SIG Sauer "continued to sell the flawed gun to the public", and that the upgrade offered "would still not fully compensate him for the significantly diminished resale value of his pistol." In March 2020, judge Joseph N. Laplante denied SIG Sauer's motion to dismiss the case. In May 2020, a trial notice was issued, with pretrial statements due on 5 October 2021; in January 2021, the due date for pretrial statements was revised to 1 February 2022. The motion for class certification was denied in 2022.
 On 30 November 2022, a lawsuit was filed in U.S. federal court alleging several P320 pistols unintentionally fired "without the trigger being pulled or deliberately actuated by the user." The lawsuit claims there have been "over 100 incidents" of such discharges, "many of which have caused severe injury." An attorney representing the lawsuit's plaintiffs, Robert Zimmerman, said it was the largest lawsuit against SIG Sauer involving the P320 on behalf of people who were injured. The twenty plaintiffs in the case (twenty individuals and about a dozen spouses) are from thirteen different states.
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 M18