Automatic and Selective-Fire Weapons: SMG, Assault Rifle, LMG

A submachine gun fires a pistol cartridge automatically or in burst mode from a shoulder-mounted or one-handed platform. The pistol cartridge (typically 9x19mm Parabellum, but also .45 ACP, .40 S+W, .380 ACP, and 9x18mm Makarov in Eastern European designs) operates at lower peak pressure than a rifle cartridge, which means that the overwhelming majority of SMG designs use simple blowback operation, the same unlocked-breech principle as blowback semi-automatic pistols. In a blowback SMG, the bolt is held closed only by spring tension and mass; at peak pressure the case begins driving the bolt rearward, extracting and ejecting the case and cycling the action. The design is mechanically simple, inexpensive, and tolerant of dirt and fouling.

The IMI Uzi (Israel Military Industries, introduced 1954) is the paradigm blowback SMG. Chambered for 9x19mm, it uses a telescoping bolt that wraps around the barrel at the moment of firing, allowing a compact overall length while maintaining a long, mass-heavy bolt. The Uzi's bolt-face mark is wide and slightly dished, with a round firing-pin impression slightly off-centre to the right. The Uzi has been used by the Israeli Defence Forces since the 1950s, was exported to the West German Bundeswehr Border Guard (BGS), was manufactured under licence in the Netherlands (FN/Uzi), and was carried by US Secret Service agents until the mid-1990s. In India, the Uzi has been used by some special units, and examples appear in the examination reference collection at CFSL Delhi.

The Sterling L2A1 (Sterling Armaments, UK, 9x19mm) began replacing the Sten gun in British service from 1953; the L2A3 (Mark 4) variant was adopted in 1956 and was manufactured in India under licence as the SAF Carbine 1A (based on the L2A1) and SAF Carbine 2A1 (based on the silenced Mark V) at the Small Arms Factory, Kanpur. The Sterling remains in service in several Indian state police forces, particularly in Jammu and Kashmir, where the older stock is still in inventory. Its side-mounted 34-round box magazine is a distinctive feature; the feeding geometry from a side magazine leaves a specific magazine-lip mark on the case body at the feed angle, a class characteristic the examiner can use to distinguish Sterling-fired cases from other blowback SMG cases.

The Heckler and Koch MP5 (9x19mm) is the most technically sophisticated SMG in widespread service and the one SMG whose case marks depart significantly from the blowback pattern. The MP5 uses a roller-delayed blowback system borrowed from the HK G3 rifle. Two rollers in the bolt head engage recesses in the barrel extension at the moment of firing, delaying the rearward bolt travel until chamber pressure has dropped to a safe level. This roller engagement leaves two oval-shaped impressed marks at the 3 o'clock and 9 o'clock positions on the forward shoulder of the cartridge case, a distinctive class feature absent from simple blowback SMG cases. The MP5 is issued to the German Grenzschutzgruppe 9 (GSG 9), the UK Special Air Service (SAS) and Specialist Firearms Officers, the US FBI Hostage Rescue Team (HBT), and numerous police Counter Terrorism units worldwide, including the National Security Guard (NSG) of India. The roller-delayed mark on MP5 cases was documented by the Bundeskriminalamt following anti-terrorism operations in the 1970s.

An assault rifle is a selective-fire weapon chambered for an intermediate cartridge and fed from a detachable box magazine. The intermediate cartridge is the defining feature: more powerful than a pistol round, less recoil-intensive than a full-power battle rifle round, and optimised for effective range to approximately 300-500 metres in individual aimed fire. Three intermediate cartridges dominate globally: 7.62x39mm (AK family), 5.56x45mm NATO (M16/AR-15 family, INSAS), and 5.45x39mm (AK-74 and later Soviet/Russian designs).

The AK-47 (Avtomat Kalashnikova 1947, Mikhail Kalashnikov, Soviet Union) uses a long-stroke gas-piston system in which the piston is physically connected to the bolt carrier and travels the full length of the operating stroke with the carrier. The rotating bolt locks with two locking lugs engaging recesses in the receiver. The AK-47's bolt-face is distinctive: the extractor is a spring-loaded claw on the right side; the ejector is a fixed rib inside the receiver, not a plunger in the bolt face. The ejector rib produces a distinctive gouge on the left side of the case head face as the case rotates away from the ejector rib during the ejection phase. This left-side ejector mark, combined with the round firing-pin impression and the extractor-claw mark at 3 o'clock, is a class characteristic of AK-pattern actions that allows an examiner to distinguish AK-fired cases from M16-fired cases before microscopic comparison begins.

The AKM (Modernised AK, 1959) is the steel-stamp-receiver, lightened successor to the milled-receiver AK-47 and is the variant most widely distributed globally. AKMS is the folding-stock version. AK-203 is a 7.62x39mm modernised Kalashnikov variant adopted by India under a 2019 joint-venture agreement between Kalashnikov Concern Russia and the Ordnance Factory Board India, with production at the Korwa Ordnance Factory (Indo-Russia Rifles Private Limited) in Amethi, Uttar Pradesh. The AK-103 will coexist with the SIG 716i in Indian service for the medium term. The AKS-74U (5.45x39mm) is a compact carbine variant of the AK-74 with a distinctive conical flash suppressor.

The M16 family uses Eugene Stoner's direct-impingement gas system and a multi-lug (7-lug in M16 bolt) rotating bolt. The M16A2 (adopted 1983) introduced the 3-round burst limiter, replacing the full-auto option available on the earlier M16A1. The M4 Carbine (shorter barrel, collapsible stock) is the current standard individual weapon for US Army and Marine Corps. The M16/M4 bolt face is distinctive: the extractor is a single spring-loaded claw, the ejector is a spring-loaded plunger inside the bolt face (rather than a fixed rib), and the firing-pin impression is round with a faint primer-safety flash-hole counterbore at the centre. The spring-loaded plunger ejector produces a different mark from the AK's fixed rib ejector: the plunger mark is a small, round impressed mark at approximately 6 o'clock on the case head face, whereas the AK's rib mark is a elongated gouge on the left face.

The INSAS 5.56mm (Indian Small Arms System) uses a short-stroke gas piston and a rotating bolt based loosely on the FN FNC design. The INSAS is selective-fire with a three-round burst option and full automatic, chambered for 5.56x45mm INSAS cartridge (functionally compatible with NATO 5.56 but with a slightly different chamber dimension that requires dimensional verification before cross-chamber firing). The INSAS bolt face has a single extractor claw and a plunger ejector similar in concept to the M16 but different in specific geometry. INSAS cases fired in the field can be distinguished from NATO 5.56 cases primarily by the headstamp (OFT + year code) and, on comparison microscopy, by the INSAS-specific bolt-face and extractor marks documented in the CFSL Hyderabad and CFSL Chandigarh reference collections.

The FN FAL (Fusil Automatique Leger, Fabrique Nationale, 7.62x51mm NATO) was the standard rifle of over 90 countries at its peak distribution. Gas-operated, tilting-bolt, with a piston rod running above the barrel in a gas tube, it presents a distinctive tilting-bolt lock mark: a forward-tilting locking shoulder mark at the 6 o'clock position on the case body, left by the rear of the tilting bolt as it locks downward. The FAL has been the standard rifle of India's paramilitary Central Reserve Police Force (CRPF) for decades in the 7.62x51mm configuration, and still appears in CRPF issued inventory. Its fired case signature is well-documented at CFSL.

The IWI Tavor TAR-21 (Israel Weapon Industries, 5.56x45mm NATO) is a bullpup selective-fire rifle adopted by the Israeli Defence Forces as a standard infantry weapon from 2009 and exported to India (in limited numbers for special units), Azerbaijan, Ukraine, and several other military users. The bullpup configuration places the action behind the trigger, shortening the overall length without reducing barrel length. The Tavor's bolt-carrier operates in a long receiver inside the stock body; its ejector is positioned for right-handed ejection by default (a left-hand variant ejects left). The Tavor fired-case mark set includes the standard rotating-bolt extractor and plunger-ejector marks but at different angular relationships from conventional designs because of the bullpup bolt-carrier geometry.

A light machine gun is a magazine- or belt-fed automatic weapon firing a full-power rifle cartridge, designed to provide sustained suppressive fire from a bipod or tripod mount. Unlike an SMG (which fires pistol-pressure cartridges) or an assault rifle (which is fired by one person standing), an LMG is operated by a two-person team (gunner and loader / ammunition carrier) and is designed to fire for extended periods, generating heat that would disable a lighter barrel.

The FN MAG (Mitrailleuse d'Appui General, Fabrique Nationale, 7.62x51mm NATO) is the most widely issued LMG globally. It uses a gas-operated, tilting-bolt system derived from the Browning Automatic Rifle. It is issued as the L7A2 to the British Army, as the M240 to the US military (M240B coaxial, M240L ground-mounted), as the C6 to the Canadian Forces, and as the MAG 58 in Indian Army service. The FN MAG uses disintegrating metal-link belts; the fired case, extracted by a spring-loaded extractor claw at 3 o'clock, shows the tilting-bolt locking recess mark and the specific FN MAG ejector mark. Because belt-fed LMGs fire continuously, the examiner at a sustained-fire scene may find dozens of cases in a linear cluster whose orientation reflects the azimuth of fire.

The Bren gun (.303 British, then converted to 7.62x51mm NATO as the L4 series) was the British Commonwealth LMG from 1938 through to the early 1980s, still in Indian paramilitary inventory in .303 form into the 1990s. The Bren is top-magazine-fed (the magazine sits above the receiver in a curved, detachable 30-round box), gas-operated, tilting-bolt. The top-feed geometry left the Bren with a characteristic case ejection pattern: cases eject downward and to the right, clustering directly beneath and to the shooter's right of the firing position. Bren-fired cases in .303 British are identifiable by the .303's rimmed case head (13.9mm rim diameter, the same as the Lee-Enfield rifle) and the Bren's specific extractor mark width.

The MG3 (Rheinmetall, Germany, 7.62x51mm NATO) is the modernised MG42 design, updated for the NATO cartridge. The MG42 was famous for its 1,200 rounds-per-minute cyclic rate during World War II; the MG3 operating at approximately 800 rounds per minute is still the primary sustained-fire LMG of the Bundeswehr and the Turkish Armed Forces (as the MG3A1). Although both the MG3 and the HK G3/MP5 use rollers in the bolt mechanism, the underlying principle differs: the MG3 is a short-recoil roller-LOCKED design where the rollers lock the bolt head directly into the barrel extension and unlock as recoil-driven cams force them inward, while the HK G3 and MP5 are roller-DELAYED blowback designs where the rollers retard, but do not lock, bolt opening against residual chamber pressure. On the fired case the MG3 typically shows recoil-extraction marks and a distinct case-body crush mark from the feed mechanism's cam-driven feed pawl, while the HK G3 and MP5 produce the twin oval roller-indentation marks on the forward case shoulder discussed in the SMG section above.

The INSAS LMG (Indian Small Arms System Light Machine Gun, 5.56x45mm INSAS) is produced at Ordnance Factory Tiruchirappalli alongside the INSAS rifle. It shares the INSAS rifle bolt group and uses a 200-round drum magazine or a 30-round box magazine. The INSAS LMG has been issued to Indian Army and paramilitary units as a squad-level support weapon; operational experience during the Kargil War (1999) documented reliability issues in cold-weather conditions. The INSAS LMG case marks are essentially identical to the INSAS rifle case marks because the bolt group is shared; distinguishing INSAS-LMG-fired cases from INSAS-rifle-fired cases from case marks alone requires comparison with a weapon-specific test fire, not class-mark analysis alone.

The three principal operating principles in automatic and selective-fire weapons are recoil-operated (energy derived from bolt-barrel recoil), gas-operated (energy from propellant gas bled from the barrel), and blowback (no mechanical lock; case pressure drives the bolt directly). Each leaves a characteristic mark set on fired cases.

Recoil operation in an automatic or semi-automatic context uses either short-recoil (barrel and bolt travel together a short distance, then unlock, as in the Browning M1917 machine gun and the M1911 pistol scale) or long-recoil (barrel and bolt travel together the full recoil distance before separating). Long-recoil automatic weapons are rare outside artillery and some early aircraft cannon designs; short-recoil is more common. The Beretta M12 SMG (9x19mm, used by Italian police and Carabinieri from the 1960s) uses short-recoil in a semi-automatic SMG variant. Recoil-operated cases show well-defined bolt-face marks because the barrel and bolt are locked together at maximum chamber pressure, and the locking surfaces leave impressed striae on the case body at the locking-lug contact points.

Gas operation is the dominant mechanism in assault rifles and LMGs. A port in the barrel, positioned 5-10 cm before the muzzle, bleeds high-pressure propellant gas into a cylinder or tube that drives a piston or the bolt carrier directly. The key forensic distinction within gas operation is between long-stroke piston (AK family: piston and bolt carrier are one unit), short-stroke piston (INSAS, HK G3, G36, SCAR: piston imparts a brief impulse then disconnects), and direct impingement (M16/AR-15: no piston; gas enters the bolt carrier directly). Long-stroke piston designs leave a consistent, slightly heavier extraction mark on the case rim because the extraction force is applied smoothly throughout the full rearward stroke. Short-stroke piston designs apply the extraction force as a sharp impulse, which can produce a slightly more abrupt extractor snap mark. Direct-impingement designs, by venting gas into the bolt carrier itself, expose the case to more carbon fouling inside the chamber during the firing cycle, which can affect the definition of chamber-contact marks on the case body.

Blowback is the simplest mechanism and is identified forensically by the absence of locking-lug body impressions, the lighter firing-pin impression depth (because the case begins moving before the firing pin blow is complete), and the characteristic slight case-head separation deformation in high-pressure blowback applications (roller-delay marks in HK-family designs being the most common example). Simple blowback designs (Uzi, Sterling, most SMGs) produce the lightest and least well-defined breech-face marks of any action type.

In the United States, the NFA 1934 makes fully automatic weapons produced after May 19, 1986 unlawful to possess for civilians (under the Hughes Amendment to the Firearm Owners Protection Act 1986). Any post-1986 full-auto weapon found at a US civilian scene has therefore either been converted from semi-automatic (using an auto sear or switch device), is a law-enforcement or military weapon, or has been illegally imported. The ATF's National Tracing Center in Martinsburg, West Virginia, can trace a weapon from its manufacturer to its first retail sale in under 24 hours for a priority case; automatic-weapon traces are treated as priority and are routed to ATF IOI (Industry Operations Inspectors) and NFA Branch simultaneously.

In the United Kingdom, fully automatic weapons have been prohibited for civilian ownership since the Firearms Act 1968 (Section 5(1)(a)). Individual possession of a Section 5 weapon requires Secretary of State authority; no UK civilian can legally possess a fully automatic weapon without this authority, and prosecutions under Section 5 carry a minimum mandatory sentence of 5 years' imprisonment. The National Crime Agency (NCA) and NABIS jointly analyse recovered automatic weapons in the UK, and NABIS maintains a separate database of automatic weapon marks from known police and military issue weapons to distinguish legitimate government weapons from illegally converted or imported ones.

In India, Prohibited Bore weapons (including all fully automatic and selective-fire weapons) are restricted to military, paramilitary, and certain authorised state police units under Arms Act 1959 § 2(1)(c). Civilian possession of a PB weapon without an Arms Act licence issued by the Union Home Ministry is an offence under § 25 of the Arms Act. Cases involving AK-47, INSAS, or other PB weapons at civilian crime scenes typically involve military or police theft, cross-border arms trafficking (from Pakistan and Myanmar in border states), or diversion from official armouries, patterns documented in NCRB Annual Crime Statistics.

At a scene where an automatic weapon has been fired, mapping the ejected case cluster in three dimensions is the first reconstruction task. Each fired case, ejected at the weapon's ejection port, follows a ballistic arc from the ejection port to the floor. The arc's shape is determined by: the ejection port velocity (related to the weapon's operating energy), the ejection port direction (fixed for a given weapon), and the weapon's orientation and movement during firing.

For a stationary shooter firing a ten-round burst from an AK-47, with the weapon held at waist height and cases ejecting at approximately 45 degrees forward-right at 2 to 3 m/s, cases land in a cluster roughly 1 to 2 metres to the right and slightly forward of the firing position, spread over approximately 0.5 m by 1.0 m. The spread within the cluster reflects the variation in ejection velocity and angle across the burst.

For a moving shooter, the case cluster stretches into a line in the direction of movement. For a weapon on a vehicle mount, the cluster can spread over many metres if the vehicle was moving. These geometric differences allow the examiner to distinguish a static firing position from a moving one and to estimate the approximate firing position when both the case cluster and the impact points on the target are mapped.

The NABIS protocol for automatic-weapon scenes in England and Wales requires the mapping examiner to number, photograph in situ, and sample every fired case individually, followed by GPS or total-station mapping of each case position relative to a fixed reference point (typically a wall corner or doorway). The mapped positions are then plotted in the NABIS case-management system alongside the comparison microscopy results, allowing linkage of a burst of cases from one scene to a burst from another scene if the comparison microscope confirms they were fired by the same weapon.

The FBI's Major Case Section uses an equivalent approach for US automatic-weapon scenes, with case positions mapped by the Evidence Response Team and entered into the NIBIN system alongside bullet comparison data. NIBIN's correlation system was extended in 2019 to accept multi-case cluster data, allowing examiners to query whether a cluster from one scene matches a cluster from another in weapon-type signature, magazine-feed direction, and extraction-mark pattern.

In India, the CFSL Hyderabad and CFSL Chandigarh have developed scene-mapping protocols for automatic-weapon incidents derived from the BSF and CRPF operational experience in Kashmir and the Northeast. These protocols are documented in internal CFSL procedure manuals and referenced in CFSL court reports, though not yet published in peer-reviewed form equivalent to NABIS or NIBIN guidance.