Patent Publication Number: US-2002007580-A1

Title: Shotgun with automatically marked ejecta

Description:
BACKGROUND AND SUMMARY OF THE INVENTION  
       [0001] The present application relates to firearms identification marking, and particularly to hand-held weapons which are modified to produce a distinctive marking on a fired bullet.  
       [0002] Background: Firearms in the United States  
       [0003] The United States is unusual among developed countries in being a heavily armed society with a high crime rate. Unlike countries which heavily restrict civilian firearms ownership, the U.S. Constitution expressly guarantees that citizens shall have the right to “keep and bear arms”; but unlike Switzerland, the U.S. has a high rate of criminal activity. A large population of heavily armed criminals, in a population which is mostly urban or suburban, presents severe dangers to public safety. It is particularly difficult to control anonymous crimes such as drive-by shootings, since the perpetrator can be gone from the scene of the crime very quickly.  
       [0004] The United States is also unique in having a judicial system under which manufacturers can be severely punished for supposed consequences of designs which were among the best known at the time of manufacture. Such ex post facto liability is a grave concern for many manufacturers, especially in the firearms industry. In such proceedings, it is common for many manufacturers to be thrown into court together, with little chance to prepare separate defenses. In particular, it is difficult for a high-end or more careful manufacturer to separate itself from the practices of a low-end manufacturer.  
       [0005] Background: Interior Ballistics  
       [0006] The present application requires some explanation of what happens inside a gun which it is fired. (This area of technology is called “interior ballistics.”) When a handgun or rifle is fired, the trigger mechanism causes a small pin (the “firing pin”) to strike against a small container of explosive material (the “primer”) in the head of the cartridge. The primer detonates, and the flame from this detonation ignites the charge of gunpowder (or “propellant”) inside the cartridge case. The propellant burns very rapidly, and its combustion produces a high pressure within the case. This high pressure exerts force on the back surface of the bullet which is held at the front of the casing, and thereby starts the bullet moving. Since the bullet makes a sliding seal to the barrel, the combustion gasses are confined to the space behind the bullet. However, as the bullet moves down the barrel, the volume which is filled with combustion gasses will steadily expand. This expanding volume would tend to reduce the pressure of the combustion gasses, except that combustion normally continues for at least part of time when the bullet is moving down the barrel. Thus the pressure behind the bullet is variable, but remains high until the bullet-barrel seal is broken (at the muzzle, or at a port if there is one). (Since the pressure is variable, it is usually specified as a peak pressure; the specified maximum pressures for common handgun cartridges will range from about 15,000 psi or less for mild cartridges such as 38 Special or 44 Special, up to about 40,000 psi or more for “hot” cartridges such as 357 Maximum, 44 Magnum, or 454 Casull. Similarly, the specified maximum pressures for rifle cartridges can be less than 30,000 for the oldest and mildest cartridges (such as 45-70), up to 60,000 psi or more for hot magnum cartridges.  
       [0007] The inner surface of the barrel, seen in FIG. 2, normally contains spiral grooves  104 , separated by high areas called lands  102 . The grooves impart spin to the bullet, to stabilize it in flight. (The grooves and lands are collectively referred to as rifling.) The bullet will be deformed, under the high pressure of the combustion gasses, to mate to the rifling. During the short time when the bullet is moving through the barrel, the twist of the rifling will give it a high rate of spin; for instance, for a bullet with a (slow) muzzle velocity of 900 feet per second and a (very slow) rifling twist rate of 1 twist in 36 inches, the bullet would be spinning at 18,000 rpm. The time a bullet spends moving through the barrel will be approximately twice the length of the barrel divided by the muzzle velocity, so (in this example) if the barrel is 6 inches long, the bullet must be brought up to 18,000 rpm within a little more than one-thousandth of one second. Spinning the bullet up this rapidly implies that the grooves are applying a large amount of torque to the bullet. The grooves must therefore have a significant cross-section. A barrel can have as few as three or as many as eleven different groove surfaces, alternating with an equal number of lands. (For example, in a Smith &amp; Wesson Model 60 revolver in .38 caliber, there are 5 grooves, each about 0.1 inches wide and 0.030 inches deep.)  
       [0008] Background: Components and Nomenclature  
       [0009] FIGS.  4 A- 4 C show examples of cartridges which can be fired by various firearms. FIG. 4A shows a typical rifle cartridge. The bullet  402  is the projectile that is forced out of the cartridge, through the barrel of the gun, and out the barrel&#39;s muzzle. The bullets are typically made of lead, usually clad with copper-based jacketing. The bullet is held by a case  404 , generally of brass. The case also contains a precisely measured charge of propellant (gunpowder)  408 . A pressure sensitive explosive, the primer  410 , is held between two portions of the case in center of the case&#39;s base  406  (or in the rim if the ammunition is rimfire). When the base portion is struck by the gun&#39;s firing pin, the primer is compressed and detonates, igniting the propellant  408  through the flash holes  412 . The charge then burns very rapidly, generating pressure which forces the bullet out of the cartridge and through the barrel. Specified maximum pressures can be more than 60,000 psi for a very “hot” magnum cartridge.  
       [0010]FIG. 4B shows a typical handgun cartridge. The parts and function are generally very similar to those of the rifle cartridge, except that the volume available inside the case  404 ′ for the propellant is much smaller. (Since the shorter barrels of handguns do not allow as much volume for the propellant gasses to expand usefully.) Also, the handgun bullet  402 ′ typically has a blunter shape than a rifle bullet, since supersonic aerodynamics are less of a concern. The example shown is “rimless,” i.e. the cartridge head  406 ′ has a rim which does not extend outside the overall diameter of the cartridge; this makes it easier to stack cartridges in a magazine. (By contrast, revolver cartridges normally do have rims.) The example shown is a 9 mm Parabellum (also known as 9 mm Luger), which is one of the most common cartridges used in semi-automatic and full-automatic handguns.  
       [0011]FIG. 4C shows a typical shotshell. The parts and function are somewhat similar to those of rifle cartridges, but the diameters are generally larger and the pressures lower. (For example, a 12-gauge shotgun has a nominal bore diameter of 0.770″.) The shotshell&#39;s head  492  is usually (but not always) metal, but the shotshell&#39;s sides  493  are typically plastic (or sometimes paper). A wad  490  is placed ahead of the powder charge. The wad is typically a polymer cup, which holds the charge of pellets in the unfired shell. The soft material of the shotshell is crimped down over the charge of pellets  494 , to hold everything in place until the shotshell is fired.  
       [0012] The wad  490  (shown in more detail in FIG. 4D) performs several functions: it not only provides a seal (at the obturating ring  490 A) for the propellant gasses to press against, but its front part  490 C also acts as a carrier for the shot, to keep them generally together until they exit the muzzle. In some cases the wad is made to crumple (in middle part  490 B), and thereby provide some shock absorption to protect the pellets against deformation. The wad also can help to prevent damage to the bore if hard (steel) pellets are used. After the wad and pellets leave the muzzle, the wad immediately slows down (due to air resistance against the leading edge of front part  490 C, which is typically slitted as shown to open up quickly). The wad normally falls to the ground within 100 feet or less. (Shotguns can also be used to shoot solid lead slugs, and in this case the wad may be bonded to the slug, to add stability in flight. In such cases the wad may not separate in flight.)  
       [0013]FIG. 3A shows the main parts of a revolver. The mechanism is complex and has many variations, so only the parts of interest for the present application will be called out here. A cylinder  301  includes chambers for a number of cartridges (typically six, sometimes five, seven, or eight). The cylinder rotates on a crane (not shown) which is pivoted to the frame. An ejector rod (not shown) at one end of the cylinder is pushed by the user to remove spent casings while the cylinder is open. When the cylinder is closed, a latch holds it in place in the frame, but allows rotation. When the user begins to pull the trigger (in a double-action revolver), the hammer rises against spring pressure, while the action also rotates the cylinder to align the next chamber with the end of the barrel  302 . (There will be a small gap between the cylinder and the end of the barrel, typically a few thousandths of an inch or less.) When the revolver is fired, the hammer  326  is allowed to strike the end of a firing pin which impacts the primer of the cartridge in the topmost chamber.  
       [0014]FIG. 3B shows the main parts of a semi-automatic handgun. Here too the mechanism is complex and has many variations (even more so than with revolvers), so only the most relevant parts will be generally described. The example shown is based on the Model 1911 design of John Browning. Before firing a cartridge is located in the chamber  311  at the end of the barrel  312 , and the barrel  312  is mechanically locked to a slide  313  which (as its name suggests) can slide in the frame  310 . The interior of the slide includes a breech face which supports the head of the cartridge. At the time of firing the internal combustion pressure not only forces the bullet forward through the barrel, but also exerts an opposite force on the head of the casing, pushing the barrel and slide backwards. The backwards movement of the barrel  312  and slide  313  unlocks the mechanical connection between them. (A wide variety of mechanisms are used for this unlocking step; in the Browning mechanism shown, a link  315  pulls the back end of the barrel downward, to separate the mating connection  316 .) The barrel  312  is only allowed to travel backwards for a limited distance, but the slide  313  continues backwards under its own momentum. As the barrel  312  and slide  313  separate, extractor and ejector mechanisms (not shown) will pull the empty casing out of the chamber  311  and throw it out of the gun. The backwards motion of the slide  313  compresses a recoil spring  317  and cocks the hammer  318 . After the slide&#39;s backward motion stops, the recoil spring  317  returns the slide  313  forward, picking up the next round from the magazine  319  and reengaging with the barrel  312  to return to firing position. In full-automatic operation (with weapons which allow this) the next round will fire immediately if the trigger is still being held down; a semi-automatic mechanism requires that the trigger be released and then pulled again to fire each round.  
       [0015]FIG. 3C generally shows the main parts of a rifle. The example shown is a semi-automatic rifle, but other common action types are bolt-action, lever, pump, break-open (double), and falling-block (single-shot). The barrel  322  is mated to a receiver  323  which contains the action. The action locks the chamber for firing, and, when trigger  324  is pulled, opens the chamber, removes the spent casing, and loads the next round. The stock  320 , generally of wood or polymer and attached to the receiver, provides a means for the gun to be held and steadied, and to minimize gun movement in recoil.  
       [0016]FIG. 3C 1  shows the mechanism of a bolt-action rifle. Trigger  324 , when pulled, releases a sear  325 . If the rifle has been cocked and is ready to fire, sear  325  immediately moves to release the firing pin  326 , and the front part of firing pin  326  strikes the primer of a cartridge in the rifle&#39;s chamber. After firing, the rifleman operates the bolt to unlock it from the receiver body and move it backward, and extractor  327  (a small claw inside the head of the bolt  328 ) grasps the base  406  of the cartridge to forcibly slide the empty case backward out of the chamber  311 . (Since the brass cartridge case has been forced against the walls of the chamber by very high pressure and transient heat, it can be much more difficult to extract than it was to insert.)  
       [0017]FIG. 3C 2  shows the mechanism of a typical lever-action rifle. In this example trigger  324 , when pulled, releases the hammer  202  to strike the firing pin  326  (if the rifle has been cocked and is ready to fire), driving the firing pin  326  forward against the primer of a cartridge in the chamber  311 . After firing, the rifleman manually opens the lever  204  (which rotates around a pivot  206 ) to lower the locking bars  208  (which in this example lock the bolt  210  to the receiver  212 ). Continued motion of the lever moves the bolt  210  backwards, and an extractor (not shown) in the bolt head grasps the head of the cartridge to pull it backwards and eventually eject it.  
       [0018]FIG. 3D shows the main parts of a shotgun. Again, a stock  320 , generally of wood or polymer and attached to the receiver  323 , provides a means for the gun to be held and steadied, and to minimize gun movement in recoil. The particular example shown is a “pump” type, where the action is operated by a sliding front piece  334 , but shotguns are also very commonly made with semi-automatic actions, and less commonly with other actions.  
       [0019] The shotgun&#39;s barrel  332  is different from a rifle barrel, since shotguns are normally not rifled. Moreover, shotgun barrels often have some “choke” (a narrowing of the bore near the muzzle) which helps to keep the pellets together. In the example illustrated, choke would be present at muzzle end  333  of the barrel  332 . (A close-up of this location is given in FIG. 3D 1 .) When the shot load reaches the choke area, the choke will tend to provide a slight radially-inward momentum component, which helps to counteract the divergence of the shot pellets which might otherwise be caused by the divergence of gas flow at the muzzle and by elastic interactions between pellets. When the wad reaches the choke, it is still under positive pressure (as evinced by the muzzle blast), and has already undergone some heating due to friction and to combustion.  
       [0020] The foregoing will help to illustrate the terminology used in this patent application. The usage of firearms terminology has varied somewhat over time, and sometimes differs between British and U.S. writers. Military usages also sometimes vary from civilian usages, especially when artillery rather than small arms is being discussed. Thus some care is needed in reading publications in this area.  
       [0021] Background: Terminal Ballistics  
       [0022] When a bullet hits something, the bullet will normally be deformed by the impact. The degree of deformation will depend on the bullet weight, construction, and velocity, and on how hard or soft the target is.  
       [0023] Bullets (other than target bullets) are normally designed to expand reliably for a given expected velocity and target type. A bullet which does not expand may simply poke a hole in the target; in hunting, this means that an animal may simply be wounded, and run far away to die alone; in police work, this means that a criminal may be able to kill police officers even after being fatally shot. At one extreme, hunting bullets used for “varmint” rifles are designed to expand very rapidly, so that small animals can be killed at extreme range. (Similarly, fragmenting bullets are marketed for home defense, so that these bullets will be less likely to penetrate walls and injure innocent third parties.) At the other extreme, very durable full-metal-jacketed bullets are used for elephant hunting, so that the bullet does not expand and stop harmlessly in the elephant&#39;s thick skin. (Full-metal-jacketed bullets are also required by treaty for military use.)  
       [0024] Thus a bullet which passes through a soft target, at less than its designed impact velocity, may not expand, and may be recovered intact. If a bullet does expand, the expansion will typically be greatest at the front: the nose of the bullet may be flattened, and/or the jacket may be partially peeled back. Such bullets can still often be used for striagraphy, but the deformation of the recovered bullet makes striagraphy more difficult. Similarly, a bullet which has ricocheted off a hard &#39;surface may still be readable, but the resultant deformation makes this more difficult.  
       [0025] Background: Forensic Bullet Identification  
       [0026] When a bullet is fired, not only will the rifling imprint a pattern on the bullet, but often small imperfections in the barrel, either from manufacturing irregularity or from irregular wear, create their own unique pattern of striations on the bullet. This fact has been used for many years to attempt to match a fired bullet to the gun which was used to fire it. A number of bullets will be fired through the suspected gun into a tank of water (or other collecting medium), creating intact models of the gun&#39;s distinct patterns. These will be examined to identify the pattern of striations which are unique to this gun. The suspected bullet or fragment can then be mounted on a comparison microscope and compared to the sample bullets. The results can be either positive (bullet X was shot from weapon Y), negative (bullet X was NOT shot from weapon Y), or inconclusive (the results are not clear). Most positive identifications are made on striations that occur in land impressions and the best marks are usually near the base of the bullets, where the seal to the barrel is greatest.  
       [0027] While bullet identification can sometimes provide clear evidence for or against a bullet coming from a particular gun, it has several weaknesses. One drawback is that you must first have the gun to be able to check against. It would be helpful to be able to know from what gun a bullet was shot without having the gun, but this has not been possible previously. Another problem is that it takes a person with a lot of experience in forensic bullet identification to properly identify matches between a bullet and a gun. The identification is currently considered to be more of an art than a science. For instance, it is possible for one section of a bullet to have a very close match with one section of a bullet from another gun, but almost impossible that they would match around the entire circumference of the bullet, so the experience of the expert is very important in deciding which are true matches.  
       [0028] In recent years the practice of striagraphy has been advanced by automatic imaging systems. In such systems an image of the striae on a recovered bullet can be captured by a computer-controlled imager.  
       [0029] Recently the BATF has proposed a national database of striagraphic images. It is hoped that such a database, if appropriately classified, would permit striagraphic matching of bullets recovered from different crime scenes. However, the inherent limitation of any such database is that the striagraphic characteristics of a weapon are not known until at least one round has been fired through the weapon and then examined. Thus striagraphic matching, of itself, is useful only where a single weapon has been used in multiple crimes, or where a weapon has been subjected to a test firing and classification.  
       [0030] Previous Attempts at Bullet Marking  
       [0031] Some attempts have been made to make sure at the outset that each firearm will leave an identifiable mark on the projectiles it shoots. U.S. Pat. No. 4,175,346 to Zemsky suggests a system wherein extra grooves are cut into the bore. This was criticized, in Atchison U.S. Pat. No. 5,685,100, as increasing resistance to the bullet, to the point where a bullet could be lodged in the barrel. (A lodged bullet can cause explosion and injuries, since the extra resistance of the lodged bullet will permit the pressure induced by the propellant gasses to rise above the maximum.) Instead, Atchison &#39;100 proposed use of microscopic markings in the bore. In an alternative, Atchison &#39;100 proposed engraving or embossing both the throat and bore, so that casing and bullet would have matching markings after each shot.  
       [0032] Previous Attempts at Marking Spent Casings  
       [0033] U.S. Pat. No. 5,758,446 to Atchison suggests a different kind of firearms identification. This patent proposes that the area around the firing pin (or even the firing pin itself) should be marked with firearm identification data, so that spent brass will thereby be embossed.  
       [0034] However, the present inventors have realized that this schema can be improved on. One shortcoming is that the brass has the thickest section at the head of the casing. Moreover, the brass at the head of the casing is already deeply embossed with a “headstamp” (identifying the manufacturer and cartridge), which reduces the space available for further identifying markings.  
       [0035] Background: Error-control Coding  
       [0036] Coded digital communication systems use error control codes to improve data reliability at a given signal-to-noise ratio (“SNR”). For example, an extremely simple form (used in data storage applications) is to generate and transmit a parity bit with every eight bits of data; by checking parity on each block of nine bits, single-bit errors can be detected. (By adding three error-correction bits to each block, single-bit errors can be detected and corrected.) In general, error control coding includes a large variety of techniques for generating extra bits to accompany a data stream, allowing errors in the data stream to be detected and possibly corrected. See generally, e.g., Lin and Costello, ERROR CONTROL CODING: FUNDAMENTALS AND APPLICATIONS (1983); Benjamin Arazi, A COMMONSENSE APPROACH TO THE THEORY OF ERROR CORRECTING CODES (1988); Michael Purser, INTRODUCTION TO ERROR CORRECTING CODES (1995); Stephen Wicker, ERROR CONTROL SYSTEMS FOR DIGITAL COMMUNICATION AND STORAGE (1995); Vera Pless, INTRODUCTION TO THE THEORY OF ERROR-CORRECTING CODES (3.ed. 1998); all of which are hereby incorporated by reference.  
       [0037] Firearm with Identifiable Projectiles  
       [0038] The present application discloses a shotgun barrel which is modified to mark the wad in a load being fired. The wad from a shotgun discharge will come down at a location which is fairly predictable with respect to the shooter and his direction of aim, but it will be far enough from the shooter that he may not see it, or may forget to pick it up. This is particularly useful for enforcement of game laws, since such wads may lie on the ground for weeks.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0039] The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:  
     [0040]FIG. 1 shows a section of the barrel of a gun, showing the disclosed innovative markers (on the lands, in this example).  
     [0041]FIG. 2 shows a section of a conventional gun barrel, demonstrating the lands and grooves of the rifling.  
     [0042]FIG. 3A shows the main parts of a revolver, and  
     [0043]FIG. 3B shows the main parts of a semi-automatic handgun,  
     [0044]FIG. 3C shows the exterior of a rifle,  
     [0045]FIG. 3C 1  shows the mechanism of a bolt-action rifle, and  
     [0046]FIG. 3C 2  shows the mechanism of a lever-action rifle.  
     [0047]FIG. 3D shows a shotgun (pump-action in this example), and  
     [0048]FIG. 3D 1  shows its choke area.  
     [0049] FIGS.  4 A- 4 C show examples of cartridges which can be fired by various firearms.  
     [0050]FIG. 5 shows a sample embodiment of a chamber marked with ridges which are parallel to the direction of extraction.  
     [0051]FIG. 6 shows a sample embodiment of a shotgun&#39;s choke area marked with identification data in the form of ridges.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0052] The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment (by way of example, and not of limitation).  
     [0053] Sample Barrel-marking Embodiment  
     [0054]FIG. 1 shows a section of a barrel as it might be seen if the barrel was cut length-wise and laid flat. Seen in this figure are two lands  102  and three rifling grooves  104 . Also seen are a series of finer coding grooves  106  which have been etched into the lands as a means of identification. If each land area has 6 possible locations  110  for coding grooves (shown as dots at the possible locations), equally spaced from the edges of the lands and from each other, the coding of the upper land can be read as 110010 (reading top to bottom), while the lower land can be read as 000100, Of course, only part of the lands for the entire barrel can be seen.  
     [0055] One example of how this coding can be used is as follows, although it will be understood that the exact method of coding can be varied over a large range of options.  
     [0056] In a sample conceptual embodiment, a total of 40 possible positions are designated for the coding bits. Of these 40 possible positions, 8 are used to designate the manufacturer (i.e. 256 different manufacturers can be designated), 4 are used for series or model identification (i.e. 16 models or series for each manufacturer), and 18 bits for a serial number (256 thousand serial numbers for each model of each manufacturer), leaving ten bits of redundancy. If we assume that the coding grooves are 1 mil (0.001 inches) deep and 5 mil (0.005 inches) wide, with a center-to-center spacing of 10 mil, it would take only 0.40 inches of land space to code this information. This is dozens of times larger than the fabrication dimensions which can be achieved by electrodischarge machining, and an even larger multiple of the dimensions which can be achieved by photolithography.  
     [0057] Thus there is ample space for the required bit density without redundancy. However, it is more preferable, as discussed below, to include additional bits to achieve a high degree of redundancy. The available dimensions permit one or two (or more) redundant bits to be added for every data bit, to produce very robust encoding.  
     [0058] In the earlier example of a Smith Model 60 in .38 caliber, the inside circumference is 0.357 inches times pi (3.14), or 1.12 inches. Thus with 5 rifling grooves, each of about 0.1 inches wide, the total width of the lands is approximately 0.6 inches, i.e. the circumference is almost equally split between lands and grooves.  
     [0059] Even for a .22 bullet (which is the smallest very common caliber), the inside circumference of the muzzle is 0.69 inches, so that if 58 percent of the circumference is available for coding, the scheme above is useable. If less than this is available, modifications can be made for smaller sized models.  
     [0060] Sample Chamber-marking Embodiment  
     [0061] In another class of embodiments, the interior of the chamber includes identification markings, whether or not the barrel or choke also include the same markings.  
     [0062] This is applicable to both rifles and pistols. In general, this embodiment is particularly useful with fast-operating actions, such as full automatic, semi-automatic, and lever actions.  
     [0063] The markings inside the chamber can be either grooves or ridges, but are preferably elongated in the direction of extraction. (This helps to mark the spent casing more reliably and along more of its length.)  
     [0064] This is particularly advantageous with blowback actions, where the spent casing begins to move while the pressure in the chamber is still very high (i.e. before the bullet has exited the muzzle).  
     [0065]FIG. 5 shows a sample embodiment of a chamber  511  marked with ridges  501  which are parallel to the direction of extraction. In this example the chamber is dimensioned for the 44 magnum cartridge, which is used both in pistols and also in small rifles. The shape of the chamber walls in this example is almost perfectly cylindrical, so the walls are show here as if they had been unrolled onto a flat surface. Preferably the ridges  501  represent a heavily redundant encoding of the identification data, as described above.  
     [0066] Sample Shotgun Embodiment  
     [0067] In another class of disclosed embodiments, a shotgun barrel is marked in its choke area (area  333  in FIG. 3D) with markers which are dimensioned to engrave the heated polymer wad as it passes by. The feature size for reliable wad marking is preferably larger (0.001 or more), but so is the available space (typically more than an inch of circumference).  
     [0068]FIG. 6 shows a sample embodiment of a shotgun&#39;s choke area  333 ′ marked with identification data in the form of ridges  601 . The walls of the choke area are conical, so the walls are show here as if they had been unrolled onto a flat surface. Preferably the ridges  601  represent a heavily redundant encoding of the identification data, as described above.  
     [0069] In a sample embodiment, the circumference of a standard 20-gauge barrel is π times 0.615″, or 1.93 inches. If ridges are specified as 0.003″ wide on 0.006″ centers, the available number of bits is 1.93/0.006=322. This conservative specification provides enough room for more than a hundred bits of data, plus very robust redundancy encoding as described above.  
     [0070] Also shown is a portion of the barrel&#39;s cylindrical portion which adjoins the choke area. In an alternative embodiment, the ridges  601  can optionally be continued down into this area.  
     [0071] Thus when the polymer wad of a shotshell encounters the conical portion of the choke, it will already have been heated by friction and combustion, and will be imprinted by the ridges in the choke area.  
     [0072] Many shotguns have removable chokes, and in this case each replacement choke is preferably marked with identification data.  
     [0073] Manufacture of Markers  
     [0074] In a first manufacturing embodiment, the markers are added after rifling with electro-discharge machining (EDM). For instance, the MG-ED72W or the MG-ED82W, manufactured by Panasonic Factory Automation Company, are capable of machining slots as small as 5 μm (0.0002 inches).  
     [0075] In an alternate manufacturing embodiment, after formation of rifling grooves, a photoresist coat is applied to the inside of the barrel, e.g. by dipping the barrel into a photoresist and allowing to dry, or by plasma deposition of a resist. In either case, an electro-optic device, such as a light-emitting diode (LED) array, is run down the barrel to expose the resist. The pattern of illumination (typically but not necessarily one-dimensional) is controlled by a code word which is different for each barrel being marked. The movement of the exposure device uses a method which combines rotation and translation to match the helical angle of the rifling, to expose the locations of the desired markers. After the photoresist is developed, an etch is applied to etch the coded grooves. It is contemplated that cylindrical lenses can be used to expose the desired areas of photoresist.  
     [0076] In another contemplated alternative manufacturing embodiment, the photoresist is exposed using a bundle of optical fibers. The advantage of this is that the exposure tool is easier to build, and the brightness of the LEDs is not constrained, if the LEDs can be mounted where they will never have to enter the bore. However, a disadvantage is that the attainable resolution is limited to a multiple of the fiber diameter. Here too the exposure tool is preferably rotated to match the helical pitch of the rifling.  
     [0077] Either of these methods of manufacturing allows the coded marking be applied electronically, so there is no change in machining movements.  
     [0078] Sample Code Embodiment  
     [0079] A simple approach to redundant encoding is to use linear combinations of codes. For example, if n identifiable bit positions are available, then the possible number of raw codes is 2 n . If the number of bits needed for identification is only k, then n-k bits can be used for redundancy. If only one bit of redundancy is available, then conventional parity checking results (and detection of one-bit errors). However, use of more bit positions provides more robust encoding.  
     [0080] The number of ways to locate k bits of data among n positions is the binomial coefficient (n,k); for the above example, where n=40 and k=30, this is equal to about 848 million. However, with more than one position for redundancy data, we can allocate the data so that ANY set of k bits will give the correct data. This can be done by specifying a redundancy matrix which provides, at each of the n locations, a different known linear combination of the k data bits. (The redundancy matrix is an n by k rectangular matrix, which specifies what linear combination of data bits goes into each of the locations.) Allocation of the code words can be done in a variety of ways, as specified in the extensive literature on coding; the presently preferred embodiment uses Reed-Solomon coding, but other coding schemes suited for forward correction of bursty errors are also suitable.  
     [0081] A particular advantage of robust encoding is that it becomes very difficult to modify the encoding in a manufactured gun: even if some of the code markings can be physically changed, the resulting pattern must still meet the constraints of the code scheme, or it can be instantly rejected as spurious.  
     [0082] Database Structure  
     [0083] The architecture described above shows how projectiles or ejecta are automatically imprinted with known weapon-identification data. This produces a massive flow of data, corresponding to all newly manufactured guns (and possibly also all replacement barrels or removable choke elements). Collection of this flow of data, for matching with evidence recovered from crime scenes, can be done in several ways.  
     [0084] Preferably manufacturers are assigned blocks of numbers to use, along with a specified redundancy-encoding scheme. When a piece of crime-scene evidence shows a recoverable identification code, the manufacturer to whom that code was assigned can quickly be queried by law-enforcement agencies to find out the original appearance of the identified weapon, and to begin the process of tracing that particular weapon through sales channels.  
     [0085] In an alternative embodiment manufacturers would report the disposition of each assigned number to a central federal database. If such a system can be implemented with infringing civil rights, it would provide even tighter tracking of weapons.  
     [0086] Alternate Embodiment  
     [0087] In an alternate embodiment, the markers are raised stripes on the land, rather than grooves etched into the lands. These would be more susceptible to erosion, but would have less chance of fouling.  
     [0088] Alternate Embodiment  
     [0089] In a further alternate embodiment, the markers are raised bumps present in only one portion of the barrel. Although the indentations made by the bumps would be partially smoothed out by the journey through the rest of the barrel, sufficient marking would be made to be detectable.  
     [0090] Alternate Embodiment  
     [0091] In a further alternate embodiment, markings are created in the grooves of the barrel, either in addition to or instead of on the lands. This embodiment is less preferable, as there is less contact pressure in the grooves.  
     [0092] Alternate Embodiment  
     [0093] In a further alternate embodiment, not only is the portion of the bullet which passes through the barrel marked, but so is the brass case which held the gunpowder and bullet. This would be particularly useful with automatic weapons, in which the brass cases are automatically ejected from the gun after each shot. This might comprise raised or grooved areas in the chamber, especially in the vicinity of the casing, where loading and unloading of the cartridge would mark the case.  
     [0094] According to a disclosed class of innovative embodiments, there is provided: A shotgun comprising: a barrel in which a shotshell, including wadding, can be fired; wherein said barrel includes mechanical features which are dimensioned to mark the wadding, with identification data for said shotgun, when the shotshell is fired.  
     [0095] According to another disclosed class of innovative embodiments, there is provided: A method of identifying the gun from which a recovered wad was fired, comprising the steps of: recovering striagraphic identification data from said wad; and looking up said striagraphic identification data in at least one database of unique identification data emplaced in shotgun chokes and/or barrels before sale.  
     [0096] Modifications and Variations  
     [0097] As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given.  
     [0098] For example, in one class of embodiments the encoding is restricted to maximum and minimum percentages of each bit. The objective here is to reduce concerns about coding-induced degradation in accuracy, and to provide further robustness in decoding the markings on a spent bullet.  
     [0099] Other encoding schemes can be used for error correction, including Hamming coding, Reed-Muller coding, BCH coding, Golay coding, Gold codes, Fire codes, tail-biting trellis codes, etc. In one contemplated embodiment, trellis coding is used to constrain the sequences of bit values used at the different positions. This has the advantage that the density of 1s can easily be constrained, to restrict degradation of accuracy.  
     [0100] In another contemplated embodiment, block turbo coding is used with 2:1 redundancy; e.g. a (90,30) block turbo code is used.  
     [0101] Since a bullet is rotationally symmetrical, it is possible to have as many duplicates of each coding as there are lands. To avoid this problem, it is preferable to encode an origin marker groove, e.g. one groove which is wider than the rest, or different in some other way, to break the symmetry and fix the rotation.  
     [0102] In a further contemplated alternative, more complicated bit-marking schemes can be used to avoid data dependence. This could, for example, be solved by coding a “0” as two thin lines, while a “1” would be machined as a single thick line.  
     [0103] For another example, it is contemplated that, in at least some alternative embodiments, the identification markings can be formed by electrodeposition rather than electro-discharge machining, or by positive resist rather than negative.  
     [0104] For another example, it is contemplated that, in at least some alternative embodiments, a mechanical operation can be used to mass-mark a batch of barrels, to thereby minimize the number of electrically or optically marked bits.  
     [0105] For another example, it is contemplated that, in at least some alternative embodiments, chamber marking can be implemented with ridges rather than grooves. In this embodiment, electrodeposition can be used to form the ridges. The advantage of this class of embodiments is that ridges will be self-cleaning. However, a disadvantage is that ridges can more easily be polished away by users, and may be more susceptible to wear. For another example, it is contemplated that barrel markings can also be implemented in designs (such as Glock™ pistols) which use polygonal rifling.  
     [0106] For another example, it is contemplated that, in at least some alternative embodiments, identification data which includes error control coding can be formed as a pattern of pits (or mesas), although this is less preferred.  
     [0107] For another example, it is contemplated that, in at least some alternative embodiments, raised markings can be formed of a hard material, such as titanium nitride, by a resist-patterned plasma deposition process.  
     [0108] For another example, it is contemplated that, in semi-automatic pistols which include fluted chamber walls, identification markings would be place on the cylindrical parts of the chamber walls, which make direct contact with the casing.  
     [0109] For another example, it is also contemplated that the described identification markings are not limited to new guns at the time of manufacture, but can also be applied to used guns.  
     [0110] It should also be noted that identification data for rifled barrels can be duplicated in different calibers (since the caliber is an identifier by itself, the same code could be used for a .38 and for a .45, without causing confusion), and can also be duplicated between models which have different numbers of lands and grooves.  
     [0111] A further contemplated advantage of error control coding schemes is that they make alteration or erasure of identification data more difficult.  
     [0112] The disclosed innovations are not all necessarily limited to the specific small-arms types described; in various embodiments it is contemplated that various ones of the disclosed innovations can be implemented in single-action revolvers, double-action revolvers, self-loading revolvers, or derringers, or in combination with blowback, delayed blowback, blow-forward, gas-operated, recoil-operated, pump-operated, lever-operated, break-action, falling-block, or bolt actions.  
     [0113] For another example, it is contemplated that, in at least some alternative embodiments, the identification markings of a given firearm can be read out, nondestructively and without firing, by inserting a soft insert. This is particularly advantageous for reading out shotgun identification data, since a game warden can be equipped with soft polymer blanks to take an impression from a gun&#39;s choke, to preserve firearm identification data in the field.  
     [0114] For another example, it is contemplated that, in at least some alternative embodiments, an extra polishing operation can be used in manufacturing, to ensure that the identification marks are not obscured by the machining marks normally detected by striagraphy.  
     [0115] The following background publications provide additional detail regarding possible implementations of the disclosed embodiments, and of modifications and variations thereof, and the predictable results of such modifications: Brian J. Heard, HANDBOOK OF FIREARMS AND BALLISTICS, (1997); T. A. Warlow, FIEARMS, THE LAW AND FORENSIC BALLISTICS (1996); John E. David, AN INTRODUCTION TO TOOL MARKS, FIREARMS AND THE STRIAGRAPH (1958); George Nonte Jr., FIREARMS ENCYCLOPEDIA (1972); Julian S. Hatcher, HATCHER&#39;S NOTEBOOK (3.ed. 1962); Julian S. Hatcher, TEXTBOOK OF FIREARMS INVESTIGATION, IDENTIFICATION, AND EVIDENCE (1935); Ian Hogg, HANDGUNS AND RIFLES (1999); Sam Fadala, RIFLE GUIDE (1993); Edward Ezell et al., SMALL ARMS OF THE WORLD (12.ed. 1983); Richard Law et al., THE FIGHTING HANDGUN (1996); Michael McIntosh, SHOTGUNS AND SHOOTING (1995); Patrick Sweeney, GUNSMITHING: PISTOLS AND REVOLVERS (1998); GUN DIGEST BOOK OF EXPLODED HANDGUN DRAWINGS (1992); GUN DIGEST BOOK OF FIREARMS ASSEMBLY/DISASSEMBLY (5 volumes); LYMAN RELOADING HANDBOOK (47.ed. 1992); LYMAN SHOTSHELL RELOADING HANDBOOK (4.ed. 1998); SPEER RELOADING MANUAL (RIFLE AND PISTOL) (13.ed. 1998); HORNADY HANDBOOK OF CARTRIDGE RELOADING (4.ed. 1991); all of which are hereby incorporated by reference.  
     [0116] None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: THE SCOPE OF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none of these claims are intended to invoke paragraph six of 35 USC section 112 unless the exact words “means for” are followed by a participle.