|the 1940 mystery writer||
_To understand the science of forensic ballistics, it helps to understand a) how a round of ammunition is constructed, b) how a gun barrel is manufactured, and c) how a gun works.
Rimfire vs. centerfire, expended shell casings
_a) The modern cartridge (another name for a round) consists of a cylindrical casing with a sealed base, usually made from brass, that’s filled with gunpowder and plugged shut on the open end with a bullet. The base includes a primary explosive charge, either rolled into the casing’s edge (rimfire) or enclosed within a tiny metal cup called a percussion cap, which is then inserted into a little recess in the brass (centerfire). Some bullets are made slightly larger than the barrel of the intended gun; some are made slightly smaller. (More on this in a bit.)
Rifling in barrel of 105mm tank gun
_b) Most gun barrels are manufactured from solid metal rods. Such a rod is first bored out and smoothed to a precise diameter, then it’s rifled to etch a pattern of spiral grooves into the interior, with raised areas called “lands” between the grooves. Gun barrels can also be forged over mandrels containing a reverse image of the rifling desired by the manufacturer. This rifling forces the bullet to whirl or spin about an axis, stabilizing its flight and improving the shooter’s ability to aim.
Note that the tools used in the boring and rifling processes are worn down a bit by each successive rod that’s transformed into a gun barrel. At the microscopic level, therefore, no two gun barrels are precisely the same.
Muzzle flash as semi-auto is fired
_c) When a gun’s trigger is pulled, the hammer or firing pin strikes the primer, setting off the explosive. This in turn ignites the gunpowder, which detonates in a controlled explosion, heats the bullet, and hurls it through the barrel into flight. Bullets that are slightly smaller than the barrel flare out and enlarge to fill the space; those that are slightly larger are squeezed to fit. In both instances, the bullet’s metal engages the rifling and is permanently marked by the interaction.
For the gun to be fired a second time (or the chamber, in the case of a revolver), the brass casing must then be removed. Some guns rely upon the explosion’s gasses to propel the brass from the breech; others use an extractor mechanism which may include an ejector, as well. Finally, some revolvers require the shooter to remove the casings from the barrel and reload manually.
Two expended slugs under comparison microscope
_This gives the forensic ballistics expert two potential evidentiary items at the scene of a shooting: the bullet and the brass casing. The bullet will carry striations from the barrel’s rifling. The casing will be marked by the hammer or firing pin, any imperfections on the firing chamber’s breech, and the individual gun’s ejector mechanism.
If a bullet strikes a hard surface (a brick wall, for example, or a bone), it can be smashed into a misshapen lump, destroying or altering the marks left by the barrel’s rifling. Therefore the brass casing can be more valuable for identifying a murder weapon, particularly if the manufacturing process left small imperfections on the breech, firing mechanism, or ejector. For this reason, criminals often try to collect and carry off expended shell casings after committing a crime, or they fire the gun through a paper bag, which camouflages the gun until it’s fired and contains the casings that would otherwise be hurled about.