Firearms identification – Concerned mainly with determining whether a bullet or cartridge was fired by a particular weapon. It is not to be confused with ballistics - which is the study of a projectile in motion. Bullet Comparisons – The inner surface of the barrel of a gun leaves its markings on a bullet passing through it. The gun barrel is produced from a solid bar of steel that has been hollowed by drilling. The microscopic drill marks left on the barrel’s inner surface are randomly irregular and impart uniqueness to each one. The manufacture of a barrel requires the additional step of impressing its inner surface with spiral grooves. The step is known as rifling.
The surfaces of the original bore remaining between the grooves are called lands. The inner diameter is known as the caliber.
Expressed in hundredths or millimeters.
.22 cal , .38 cal and 9mm.
Before 1940 barrels were rifled by having one or two grooves cut at a time with steel hook cutters. Were known as right hand or left hand grooves.
As newer techniques were developed as in the use of a broach cutter. A broach cutter cuts all the grooves at once as it is pulled and rotated through the barrel. The second technique developed involved no cutting. A “button” or steel plug impressed with the desired number of grooves forced under extremely high pressure is forced down the barrel. Last technique is known as the mandrel rifling or hammer forging process. A mandrel is a rod of hardened steel of the opposite of the intended lands and grooves. It is inserted down a slightly over sized bore.
The barrel is compressed with hammering or heavy rollers to impress the reverse of the rod into the barrel. Certain weapons can be identified by the specific number or lands and grooves they are manufactured by. .32 cal. Smith and Wesson revolvers have a right twist with five lands and grooves. .32 cal. Colt have a left twist with six lands and grooves. No two rifled barrels, even those manufactured...
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