. "2 Firearms and Ammunition: Physics, Manufacturing, and Sources of Variability." Ballistic Imaging. Washington, DC: The National Academies Press, 2008.
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Ballistic Imaging
lubricant is usually a wax or heavy grease type; due to its placement, it must be a substance that will neither contaminate the powder nor react with lead or copper plating.
2–B THE FIRING OF A WEAPON: INTERNAL BALLISTICS
The general concept of “ballistics” can be divided into separate stages; see Box 1-1. External ballistics (the flight path and behavior of the bullet between its exit from the barrel and its arrival at its target) and terminal ballistics (behavior of the bullet on striking a target) are both critical to complete firearms investigations.
Our primary focus is on internal ballistics—the actions that occur between the pulling of the trigger and the bullet’s exit from the barrel of a firearm. Internal ballistics is “a series of actions or operations that every firearm must go through, whether .22 caliber revolver or a .50 caliber machine gun,” all of which occur in a time span on the order of 0.003 seconds (Rinker, 2004:1, 2). The trigger pull starts the mechanical process of allowing the firing pin to strike the primer of the chambered cartridge. The pressure from the firing pin creates a dent in the primer surface of the cartridge; more significantly, it causes a small explosion, the heat from which passes through the hole in the primer cap and into the main body of the cartridge. There, the charge of powder burns rapidly in a confined space, converting from a solid to a gas and exerting great pressure against all surfaces. “When the pressure has built up to a sufficient level, known as short shot, the bullet will start to move because the pressure is greater than the holding force of the case neck.” As the powder burn continues, “the pressure increases and the neck and body walls of the case expand to meet and grasp the inside chamber walls,” creating a seal and increasing the pressure acting on the bullet’s base, propelling it forward (Rinker, 2004:1). The bullet, being slightly larger than the barrel diameter, is forced to seat into the rifling (the lands and grooves) on the bore of the barrel, picking up rotation as it passes down the length of the barrel.
While this sequence of events drives the bullet through the barrel and out of the firearm, forces are also at work on the head of the cartridge. Hatcher (1935:270, 272) describes the processes for a centerfire cartridge:
When a primer is struck by the firing pin, the very brusque and powerful mixture that it contains explodes with violence, [causing the flame that ignites the powder charge]. But the explosion of the primer mixture also reacts in a backward direction onto the primer cup itself, and blows it part way out of the primer pocket, unless the primer is strongly crimped in place, as is done with some kinds of rifle ammunition. Then when the main charge ignites, the powder pressure inside the case forces the case
