Module 4 - Forensic Ballistics

The term revolver is derived from the fact that individual bullet cartridges (bullets or rounds as they are commonly known) are loaded into a cylinder that revolves slightly with each pull of the trigger. The cylinder advances so that the next bullet is brought into line with the opening of the barrel.

As the firing pin strikes the bullet’s primer (located at the centre of the base of the cartridge), the gunpowder in the cartridge casing explodes, forcing the bullet out of the cylinder, into the opening of the barrel, down the length of the barrel, and out the muzzle.

A muzzle blast can be quite substantial in revolvers, especially where the cylinder lines up with the opening of the barrel. A revolver typically holds six cartridges in one of various calibres such as .22, .38, .357, and .44 (spoken as twenty-two, thirty-eight, three fifty-seven, and forty-four). The calibre of a firearm refers to the diameter of the bullet, usually measured in tenths of an inch. Therefore, a .38 calibre revolver fires a bullet that is 0.38 inch in diameter. Rather than refer to this diameter in metric units, the firearms industry has retained these commonly understood terms.

The ballistic property of revolver ammunition varies by calibre. Typically, .22 and .38 calibre ammunition have low muzzle velocities and limited amounts of kinetic energy. The structure and design of revolvers, specifically because of the use of a cylinder, result in significant loss of kinetic energy and muzzle velocity due to the escape of gases around the cylinder and barrel opening when the gun is fired.

Revolvers remain popular among collectors and people who like to target shoot as a hobby. Law enforcement agencies have generally discontinued their use of revolvers because of low ammunition capacity, lengthy reloading time, and inadequate muzzle velocity.

The Pistol

A pistol is characterized by semi-automatic features in which a slide sits on top of the frame. Bullet cartridges are loaded into a holding device called a magazine that is inserted into the handle of the pistol. The barrel sits inside the slide. Each pull of the trigger and discharge of a cartridge produces a recoil force that moves the slide back drawing a new bullet into the opening of the barrel. The slide then returns to its forward position, locking the bullet into the barrel. When the trigger is pulled again, the process repeats. Pistol bullet velocity can be considerably higher than a revolver’s velocity because the hot gases emerging from the bullet casing are contained within the barrel of the pistol, thereby increasing the pressure acting against the bullet as it moves down the barrel and out the muzzle.

The unique design of semi-automatic pistols allows for faster reloading times, higher firing rates, and higher magazine capacities (typically 10 to 16 bullets per magazine). The trigger can be pulled in rapid succession while keeping the pistol on target. For example, a person using the .40 Glock pistol is able to fire fifteen rounds of ammunition continuously before loading a new magazine.

Pistol ammunition varies in calibre, with 9 mm, .40, and .45 being the most common. Law enforcement agencies across North America typically use 9 mm or .40 calibre pistols although .45-calibre pistols are sometimes used.

While occasionally prone to jamming if not properly maintained, automatic weapons have increased among criminals in recent decades. Therefore, law enforcement agencies have adopted semi-automatic pistols because of their large magazine capacity. In addition, police are now using hollow point bullets. Recent trends include the use of semi-automatic rifles (carbines) by patrol officers in response to the increased firepower of some criminals. The public became aware of this increased firepower because of such high profile incidents as the 1994 North Hollywood shootout in Los Angeles.

Semi automatic pistol Cartridges