From the beginning of firearms, it was a fascination that how a firearm works. From simple Roman Candles to present day Fully Automatic Assault Rifles, the firearm industry has seen a change beyond belief. As a layman when it is a curiosity about the features and capability of a particular firearm, there is an implemented interest in the mechanism forensically. Initially, the mechanism of firearms was very elemental type and much simple bearing only some easy fundamentals.
But nowadays, modern scientific techniques and advanced technology made firearm development more sophisticated. Where, the initial firearms used to fire only a single projectile and that too with the help of manual loading, the present-day firearms don’t need to be loaded manually with added modifications.
The barrel of the firearm provides space for the expansion of gases and housing for the cartridge in most firearms. The chamber is on one end of the barrel. The bore and the chamber are connected in many firearms through a tapering surface called leed.
The bore of a barrel is also called a caliber or gauge. It is measured in millimeters in most parts of the world. In the united kingdom and some commonwealth countries, it is measured in thousands of an inch. It is measured in hundredths of an inch in unites states of America.
The caliber of a firearm is usually nominal. Its exact measurement frequently differs from the nominal bore size. Thus, the diameter of the bore of thirty-eight revolvers is about 0.334 inches only. The actual diameter of a bullet also differs from its nominal caliber.
When a bullet is pushed out from the cartridge case to barrel and before it sets into the grooves, it slips over the leed and gets imprinted with the slippage marks. They are often useless for identification purposes but the bullets which fit loosely in the barrel and do not get imprinted with the barrel marks may be identified sometimes in respect of the suspected firearm through the slippage marks left by the leed, by special techniques.
The action consists of the mechanism for loading, firing, extraction, and ejection of the cartridges, the magazine, and the safety devices if any. The arrangements for the location of the devices vary in different firearms.
The receiver is the frame that contains the operating (or moving) parts of the firearm and includes the breech, which is usually the part of the barrel that includes the chamber into which a cartridge or projectile is loaded, and the firing mechanism.
In the firearm, the stock holds the other parts in position and provides support for firing purposes. The stock also carries the magazine in automatic and semi-automatic pistols.
On the other hand, a magazine is a container that holds the cartridges. Sometimes magazines themselves are loaded with cartridges by clips. On most firearms, a magazine is detachable and replaceable.
- Firing Pin or Striker
It is the part of the firearm which, on pressing the trigger, moves forward and strikes the cartridge with force. It is located in the breech block in shotguns, in the bolt heads of bolt action rifles, and in the moving breech block in automatic and semi-automatic firearms. It forms part of the hammer in revolvers. It is the most important part of the identification of fired cartridges. The firing pin indentations are invariably present in all fired cartridges. The indentations alone permit the identification of firearms in about ninety-five percent of cases.
- Breech Face
The area surrounding the firing pin hole in the breech face. It steals the barrel from the chamber side and holds the cartridge cases. When the latter is pushed back due to expanding gases on firing. It comes in close contact with the base of the cartridge and the irregularities of its surface are imprinted on the cartridge.
The breech face is the second most important part of a firearm from the point of view of identification of a fired cartridge in respect of the firearm which fired it.
The marks are particularly important in cartridges developing high pressure. The cartridge fired from standard shotguns in most of the cases, does not carry sufficient breech face marks. On the other hand, rifles developing high pressures imprint conspicuous breech face marks.
The chamber surface of a firearm has microscopic imperfections. These irregularities on the surface usually do not mark the fired cartridge because the diameter of the chamber is larger than the diameter of the cartridge.
The cartridge, therefore, does not come in very close contact with the surface of the chamber even after expansion. Sometimes there may be some abnormality or some extraneous material deposit in the chamber which may reduce its effective diameter. If a firearm is made by a local blacksmith, the required tolerance in the chamber may be absent and the cartridge is housed tightly in the chamber.
On firing, the cartridge case expands and comes in close contact with the chamber surface, and gets imprinted with the chamber marks. Chamber marks are frequently found in improvised firearms.
- Extractor and Ejector
These parts of a firearm are of various designs and forms. They come in contact with a cartridge case fired through the firearm and imprint their surface irregularities on the same. The marks are often insufficient for identification purposes. They are characteristics only in a few cases to allow the identification of firearms. The type of ejector is a spring-driven plunger in the face of the bolt. The plunger forces the case to flip out of the port when the case is free of the chamber.
- Choked Barrels
When the diameter of a barrel of a shotgun is the same throughout the barrel bore, it is called the true cylinder. The bore of a shotgun is often reduced near the muzzle end. The diameter near the muzzle end is slightly smaller than the diameter of the bore of the rest of the barrel. The barrel is said to be choked. The choke is full, half, quarter, or improved cylinder if the diameter at the muzzle end is reduced by about one, half, one quarter, and one-tenth of a millimeter respectively. The choke increases the effective range of a shotgun by increasing the concentration of pellets per unit area at the targets.
For example; for every hundred pellets fired, seventy pellets will fall in a circle of seventy-five centimeters at a range of thirty meters in a fully choked barrel. The number of pellets falling in an equal circle under similar conditions for a true cylindrical barrel is forty only.
Firearms can be classified into the following types based upon the mechanism:
1) Manual Mechanisms
The important feature in the manual mechanism firearms is that the cartridge is loaded to the chamber manually after firing of every round. In these types of mechanisms, the cartridge-filled magazine is locked into the firearm. If it supports an auxiliary feeding mechanism, from which every single cartridge shall be loaded and fired from the firearm.
2) Automatic Mechanisms
The action of an automatic firearm is capable of harvesting the excess energy released from a previous discharge to feed new ammunition round into the chamber, and then ignite the propellant and discharge the projectile either bullet, shots, or slugs by delivering a hammer/striker impact on the primer.
The Mechanism of Firing
The hammer or the striker is released with considerable force when the trigger of a firearm is pressed. The striker compresses a pressure-sensitive material that is contained in the percussion cap. This develops a hot piercing flame and helps to ignite the propellant charge. The charge quickly transforms into gas. A large volume of gas is produced and develops very high pressure due to the limited space in the cartridge case. The pressure so developed forces out the bullet or the shot charge through the barrel towards the target. While the bullet moves forward the cartridge case is pushed backward. The case, therefore, comes in contact with the firing pin and the breech face of the breech block and picks up marks from their surface. The cartridge is also expanded all around due to the tremendous pressure developed in it. It comes in intimate contact with the chamber and sometimes may carry chamber marks. The fired cartridge case is extracted out of the chamber by an extractor and then ejected out of the gun by an ejector.
In the process, both extractor and ejector are likely to leave marks on the cartridge case. All these marks are very useful which can afford the identity of the or the cartridge case with the concerned firearm.
Mechanism of Bolt Action
Bolt action weapons are controlled by a bolt that opens and closes. It is possible to lift the bolt. It aids in determining whether or not the chamber is loaded. The chamber is usually loaded and a cartridge is expelled when the bolt is opened.
A spinning bolt slips into an extension to the barrel in bolt–action guns. This is the same mechanism that is used to lock a door with a turn bolt. When we push the bolt forward, it cocks the striker or firing pin by bringing the bolt face into the battery with the breech end of the barrel. Then, using the bolt lugs to engage with slots in the barrel extension, spin the bolt and secure it into position. The firing pin, a spring, and an extractor are all enclosed in a locking breech block in this sort of mechanism.
Lever Action Mechanism
A handle is fixed below the trigger in the lever mechanism. It acts as the trigger guard in the firearm. When it is pushed forward, a rod is pulled backward. It extracts the fired cartridge and cocks the firearm. Simultaneously, the carrier bolt is pushed upward which carries a live cartridge. The rod places the live cartridge in the chamber when the lever is brought to its normal position. The carrier block takes its original position and the firearm is ready to fire. Once a lever-action gun is cocked, the only way to unlock it is to clutch the hammer and squeeze the trigger. In contrast to bolt-action, semi-automatic, or selective-fire weapons, most lever-action firearms are rifles, but some lever-action shotguns and a few pistols have also been produced.
The Winchester Model 1873 rifle is One of the most celebrated lever-action firearms. Due to the sophisticated rate of fire and shorter overall length than most bolt-action rifles, lever actions have persisted prevalent to this day. It is for sporting use, specifically short- and medium-range hunting in forests, scrub, or bush land.
Mechanism for Slide Action
Slide-action weapons are sometimes known as Pump-action or Trombone Action weapons. In this type of mechanism, the breech block is linked to a moveable fore-end through operating rods. The mechanism that holds the breech block to the barrel is freed when we draw back the fore-end. while the fore-end is dragging to the back of its travel range, then pushing it forward The action is cocked after the empty cartridge case is evacuated and a new round is placed into the chamber.
Shotguns with pump actions are prevalent. The mechanism in this shotgun opens the chamber ejects the empty cartridge case and picks up the next round from the magazine by pulling the fore-stock to the rear. It propels it forward once more, chambering a new round. Tubular magazines are most commonly used, but instances with detachable box magazines have been identified.
Break Action Mechanism
The barrel of a break-action firearm is hinged and can be “broken open” to reveal the breach. The knob can be found beneath the barrels. The barrels swivel on this pin, which is threaded through it. On the opposite side of the knob, there is an aperture. The operator turns the lever in front of the trigger guard to close the barrels back in place. A projection is pushed into the slot. It also prevents the barrels from rising. When the barrels are tilted upwards, the user can remove the old cartridges, replace them, and then close and lock the mechanism. For two-barrel configurations, multi-barrel break-action firearms are normally split into over-and-under or side-by-side configurations, or “combination gun” when mixed rifle and shotgun barrels are employed.