Firearms: Types, Classification, Ammunition and Compositions

A firearm is a barrelled weapon that drives a projectile out of its bore by the rapid expansion of gases from a confined burning propellant charge. The statutory definition in India sits at Section 2(1)(e) of the Arms Act 1959 and is deliberately wide: it folds in barrels, locks, magazines, machinery for manufacture, and any accessories, so the law catches both the assembled weapon and the parts traffic that feeds it. Ammunition is defined separately at Section 2(1)(b) and covers cartridges, shot, shell, fuses, primers, propellants and the assembled rounds the IO seizes from a scene.

Once the firearm and the ammunition are defined, six terms carry the rest of the subject. The cartridge is the complete round loaded into the chamber. Inside it sit four parts: a case (usually brass), a primer at the head, a propellant charge in the middle, and the projectile crimped at the mouth. The projectile is the object that leaves the barrel; the propellant is the charge that drives it; the primer is the small impact-sensitive explosive that ignites the propellant when the firing pin strikes.

Firearms classify along four independent axes. Combine the axes to describe any given weapon precisely.

By barrel (rifled versus smoothbore). A rifled barrel has spiral grooves cut into the bore that spin the bullet on its long axis for gyroscopic stability. Handguns (revolvers and pistols) and rifles use rifled barrels. A smoothbore barrel has no rifling and is the norm for shotguns, which fire either a swarm of pellets (birdshot or buckshot) or a single solid projectile (slug). Forensic significance: rifled barrels imprint land and groove striations on the bullet that allow individual identification (the classical comparison-microscope match used in the firearm injury examinationat the autopsy); smoothbores do not.

By action. The action is the mechanical group that loads, fires and ejects a cartridge. The principal kinds are bolt action (rifle, manual operation of a turning bolt), lever action (cowboy rifles, lever under the trigger guard), pump or slide action (shotguns, fore-end pumped back and forth), semi-automatic (one shot per trigger pull, gas or recoil cycling the action), fully automatic (continuous fire while the trigger is held, generally military), break-open (single or double-barrel shotguns and rifles where the barrel hinges down), and revolver (the classical wheel of chambers in a cylinder that rotates with each trigger pull).

By loading. Muzzle-loaders are loaded by pushing powder and projectile down the front of the barrel; they are historical and only show up in collector cases. Breech-loaders are loaded from the rear of the chamber and are universal in modern firearms.

By use. Military arms (assault rifles, machine guns, sub-machine guns), sporting arms (hunting rifles, shotguns, target pistols), civilian self-defence arms (small-caliber pistols and revolvers), and in India the further legal split into prohibited bore (PB) and non-prohibited bore (NPB) discussed in the next section.

The Indian legal framework for civilian firearms is the Arms Act 1959, modernised by the Arms Rules 2016 (which replaced the 1962 rules) and amended by the Arms (Amendment) Act 2019.

Prohibited bore (PB). Calibres classed as PB include.303 British,.455,.380 ACP service variants, 9 mm Parabellum, 7.62 mm and certain other military and police calibres. PB licences are heavily restricted, generally limited to serving and retired armed-forces personnel, state-police authorised personnel, and a small set of civilians under explicit central-government authorisation.

Non-prohibited bore (NPB). Calibres classed as NPB include.22 LR,.32,.315 sporting rifle,.380 (civilian variant in some readings) and the shotgun bores.12,.16,.20 and.410. NPB licences are issued by the District Magistrate under conditions set by the Arms Rules 2016, with renewal cycles, secure storage and ammunition-purchase ceilings.

Licence categories under the Arms Rules 2016.Form III is the standard NPB licence for self-defence, crop protection, sport and personal safety. The 2019 amendment lowered the maximum personal holdings from three to two firearms and raised punishments for illegal manufacture and trafficking.

Forensic significance: the bore stamp on a seized cartridge case and the headstamp on the rim are the first evidence the SOCO team logs against the licence record. The seizure workflow uses the same crime-scene tools kitdeployed for any physical-evidence collection at a violent scene.

A cartridge is the complete round of ammunition, designed to be loaded into the chamber as a single unit. It has four parts assembled in this order, head to mouth.

Case. Usually drawn brass, sometimes steel or aluminium for low-cost or military rounds. The case holds the propellant, seats the primer at the head and seats the projectile at the mouth. Cases classify by rim shape: rimmed (a flange wider than the body, used in revolvers and many shotguns), semi-rimmed, rimless (the head diameter equals the body, used in most automatic pistol rounds for clean magazine feeding), belted (a reinforcing belt forward of the extractor groove, used in high-pressure magnum rifle rounds) and rebated.

Primer. A small cup at the head of the case, struck by the firing pin to ignite the propellant. Two configurations: Boxer (single central flash hole, anvil built into the primer cup, dominant in modern Western and Indian commercial ammunition because spent cases are easy to reload) and Berdan (two or three small flash holes, anvil built into the case head, common in European military and Russian-pattern ammunition, awkward to reload).

Propellant. The main chemical charge that burns to generate the propelling gas. Black powder in historical arms; smokeless powders (single-base, double-base, triple-base) in everything modern. Composition detail in the next section.

Projectile. The object the cartridge launches: a single bullet in rifled-arm ammunition, multiple pellets or a single slug in shotgun shells.

Propellant chemistry has a fixed, verifiable composition for each generation.

Black powder. The historical propellant, in use from the 14th century until the 1880s. Composition is potassium nitrate (KNO3), charcoal (C) and sulphur (S) in the classical ratio 75: 15: 10 by mass. Burns rapidly with a large volume of solid combustion products, producing the characteristic dense white smoke and the heavy fouling on the bore. Still used in some traditional and ceremonial arms, in pyrotechnics, and (rarely) in country-made katta ammunition seized in India.

Smokeless powders. Three generations, all based on nitrocellulose (NC) as the energetic backbone.

• Single-base smokeless powder. Nitrocellulose only. The first smokeless propellant (Poudre B, France, 1884). Used in shotgun shells and many handgun and rifle rounds.
• Double-base smokeless powder. Nitrocellulose plus nitroglycerin (NG). Higher energy than single-base. Used in pistol, rifle and some shotgun loads, and in many cordite formulations (the original Lee-Enfield.303 propellant was a cordite double-base composition).
• Triple-base smokeless powder. Nitrocellulose plus nitroglycerin plus nitroguanidine. Lower flame temperature and cooler burn (nitroguanidine adds gas without adding heat), which reduces barrel erosion in large-calibre artillery. Almost exclusively a military and naval artillery propellant.

The forensic significance is twofold. First, the burn chemistry generates the propellant residue (nitrites, nitrates, partially burned NC particles) that the Modified Griess and Walker filter-paper tests detect when the lab is determining the range of firing. Second, the propellant flake morphology under stereomicroscopy (disc, ball, cylindrical, perforated) is a class characteristic that ties an unfired cartridge in the suspect's possession to a fired cartridge case at the scene.

The primer is the smallest component of the cartridge but the most consequential for the forensic chemistry that follows. The classical priming composition is a three-metal mixture.

Classical Sinoxid-type primer (most commercial centrefire ammunition). Lead styphnate (the initiating explosive, sensitive to mechanical impact) plus barium nitrate (the oxidiser, supplies oxygen to the burn) plus antimony sulphide (the fuel and friction sensitiser). When the firing pin strikes, the primer detonates and produces a hot gas-and-particle cloud rich in lead (Pb), barium (Ba) and antimony (Sb). These three elements travel out of the muzzle, the breech and the cylinder gap with the bullet and the propellant gases and deposit on the shooter's hands, sleeves, face and the target. The Pb-Ba-Sb three-component particle, recovered on an adhesive lift and identified by SEM-EDX, is the gold-standard signature of primer-derived gunshot residue. This is the chemistry that the GSR-analysis bullet in this unit builds on.

Modern lead-free primers (Sintox and equivalents). Driven by occupational lead exposure on indoor ranges and military training facilities, lead-free primers replace lead styphnate with diazodinitrophenol (DDNP) or tetrazene, and replace barium nitrate with zinc peroxide or strontium nitrate. The forensic challenge is that the classical Pb-Ba-Sb SEM-EDX signature no longer fires, and the lab has to look for alternative markers (Ti, Zn, Sr, K, Si) and the morphology of the residue particle. Indian commercial and police ammunition remains overwhelmingly Sinoxid-type, so lead-free primers are an emerging casework consideration in Indian SFSLs rather than a current routine.

The projectile is the terminal component and the most forensically variable part of a round.

Bullet types (rifled-arm projectiles).

• Lead round nose (LRN). Soft lead, rounded nose, low velocity, deforms readily. The classical.38 Special revolver round.
• Full metal jacket (FMJ). Lead core sheathed in copper or cupro-nickel. Holds shape, deeper penetration. Required for military use under the Hague 1899 declaration.
• Jacketed hollow point (JHP). FMJ with a nose cavity that mushrooms on impact. Civilian self-defence.
• Soft point (SP). FMJ with an exposed lead nose. Hunting, intermediate expansion.
• Ballistic tip. Polymer-tip insert over a hollow cavity, better aerodynamics, reliable expansion.
• Armour piercing (AP). Hardened steel or tungsten penetrator core. Military.
• Tracer. Pyrotechnic base compound that burns in flight to mark trajectory. Military.
• Frangible. Compressed metal powder, disintegrates on a hard target. Indoor-range use.

Shotshell ammunition (smoothbore projectiles). Shotshells contain a propellant charge, a wad, and either a swarm of pellets or a single slug. Pellet sizes are birdshot (small, numerous, sizes 7 through 9) and buckshot (larger, fewer pellets, sizes 4 through 000). Gauge is the inverse bore measure (the number of lead spheres of bore diameter that weigh one pound), so.12 bore is larger than.20 bore. The.410 is sized in inches, not gauge.

Caliber notation reads in inches (leading decimal) or in millimetres. Common calibres in Indian forensic casework include.22 LR,.32,.315 (the Indian-made sporting rifle),.38 Special,.380 ACP, 7.62 by 39 mm (the AKM and SKS round), 9 mm Parabellum, and shotgun bores by gauge (.12,.20) or inches (.410).

Indian ordnance manufacture sits with the corporatised successors of the old Ordnance Factory Board:Munitions India Limited (MIL) for ammunition, explosives and rockets, Advanced Weapons and Equipment India Limited (AWEIL) for small arms and large/medium calibre weapon systems, and Yantra India Limited (YIL) for components. The headstamp on a fired case carries the calibre, manufacturer code and lot year, which the SFSL ballistics analyst uses to trace the round back to a factory batch, then to the dealer and the licensed buyer.