Firearms Notes Forensic Science

Categories of Firearms Shotguns- gauge Rifles-caliber Handguns-caliber Rifles and handguns are rifled (have spiraled grooves built into barrel)

Gauge Gauge - the system used to determine the interior diameter of the barrel of a shotgun. If you take one pound of lead and divide that pound into 16 balls of equal size, the diameter of those balls will fit a 16 gauge shotgun. (it is the same inner diameter as a 16 gauge shotgun)

Caliber Caliber - the system used to determine the bore diameter of a handgun or rifle – generally a measurement in either mm or inches. Bore diameter - the distance from land to land in the inside of a rifled barrel. Lands – the higher areas on the inside surface of the barrel

Lands and Grooves Cut or pressed, the entire length of a barrel, into the inner surface of a barrel in a twisting motion. Lower areas are grooves Higher areas are lands

Rifling Rifling - a “cut” made down the center of the barrel of a rifle. Purpose - The “cut” is made in a twisted pattern to cause the bullet to spiral as it moves through the air.

Rifling Cont’d A rifle’s “Twist” is the barrel distance for one spiral to be completed. Can vary from 8 to 20 inches. How much twist a gun has depends on the type of ammunition to be used (mass of bullet and amount and type of powder) The number of grooves inside the barrel can also vary. From 3 to 8 grooves For example, rifle can have a 4 groove 14 twist.

Anatomy of a Firearm Breechblock Supports base of the cartridge in the chamber Often finished by hand filing characteristic striations on this part contact the back of the cartridge leaving individualizing markings on the cartridge base

Anatomy of a Firearm Firing Pin Strikes the cartridge primer to initiate the firing process Finished on a lathe or filing by hand Can transfer striations to the soft metal of the primer cap

Firearm Anatomy, Cont. Extractor and Ejector extracts and ejectors a spent cartridge from the chamber Metal parts of extractor and ejector can leave individualizing markings on cartridge cases All of the moving components contact the cartridge rather than the bullet and can leave useful impressions on shotgun shell cartridges

Ammunition A cartridge (ammunition) is made up of gun powder, a primary explosive and a bullet all held within a cartridge case

What Happens When the Trigger is Pulled? 1. Pulling the trigger causes the firing pin to strike the back of the primer cup, leaving the impression of the firing pin on the back of the primer cup. The primer cup contains a primary explosive. (A primary explosive is one that is sensitive to friction, heat and or mechanical shock.) 2. When the primary explosive receives this mechanical shock it explodes.

Pulling the Trigger Cont’d 3. This explosion causes the gun powder in the cartridge case to ignite. The burning of gun powder (KNO3 + sulfur + carbon) produces the gases CO2, nitrogen oxides and SO2. 4. The gases, produced in this reaction, take up greater space causing the bullet that is in front of them to be forced forward through the rifled barrel of the gun. Rifling marks and striations from the inside surface of the barrel are impressed upon the sides of the bullet.

Action and Reaction For every action there is an equal and opposite reaction. When the bullet is pushed forward, it pushes back on the cartridge case and the gun itself (recoil). The cartridge case is pushed back against the breechface (rear wall of the firing chamber) of the gun. The striations found on the breechface are impressed upon the back of the cartridge case.

Removing the Cartridge Case The cartridge case is then removed from the firing chamber by an extractor. The extractor, a metal device, leaves a mark on the edge of the cartridge case rim. The ejector then pushes the cartridge case out of the firearm, leaving its mark on the side of the cartridge case. These marks, made by the extractor and ejector, will match the marks made on other cartridge cases fired by that same firearm.

Class vs. Individual

Class vs. Individual Breechface marks, firing pin marks and the striations found on the sides of the bullet have individual characterisitics. Rifling marks on the bullet only show the number of grooves and information that can give the twist. This is class information only. It can sometimes tell the manufacturer of a firearm.