ANALYZING FIREARMS EVIDENCE

Analyzing Firearms Evidence DEFINITION Forensic Firearms Examiners are specially trained to analyze weapons, bullets and ballistics Ballistics is how bullets behave after being fired from a gun

What do they do? Determine type of weapon Estimate distance between gun muzzle and victim Estimate trajectory Match bullet or shell casing to a particular weapon

How Do Guns Work? Firing pin strikes a cylinder of primer in the shell of bullet Primer gets ignited Ignited primer causes gunpowder in shell to ignite The force of this pushes bullet out barrel with tremendous velocity

Extracting Info from Ammo HANDLING BULLETS Take care to not damage/alter/scratch them - Might have trace evidence attached Bullets can tell us what weapon was used in a crime

TYPES OF BULLETS Lead - soft, low velocity, deform & fragment a lot 2) Lead Alloy - harder, high vel, deform & fragment less Semi jacketed - hollow pt type deform & fragment more Full metal jacket - greater penetrating power “cop killers” Frangible - designed to break apart/prevent ricochet

Determining Caliber & Gauge Caliber is the measurement of the internal diameter of the barrel In US, the caliber is measured in inches ex. A .38 caliber handgun has a barrel with an internal diameter of 0.38 inches In shotguns, the smaller the #, the wider the barrel The size of shotgun pellets tells you about the ammunition used. The larger the pellet, the smaller the number ex. Number 8 shot is smaller than Number 4

Shell Casing is what remains after firing What do they tell us? -The impression left by firing pin (gun type) -Breechblock patterns (impression) -Headstamps (left by manufacturer) -Extractor and ejector marks (unique scratches)

Comparing Rifling Patterns Most guns are rifled: spiral grooves are etched into inside barrel to make bullets spin (spinning bullets = more accurate bullets) In etching, lands are high parts, grooves are the indentations Each gun type has General Rifling Characteristics (in FBI Database) Can be used to determine the type of gun, and with more testing, a specific gun

Reading the Ridges When a bullet is fired, the lands and grooves mark the bullet. These markings are called striations. They are linear and parallel to the long axis of the bullet. Silencers can also leave markings on bullets Bullets fired from the same gun share the same striation patterns. But 2 bullets don’t have to match in every detail to be considered solid evidence. (Only need 3 matching striations, why?)

GUNSHOT RESIDUES When a bullet is fired, gases and particles escape from the gun. These particles, called gunshot residue (GSR) cling to the shooter’s hand, arm, clothing, face, and hair. (In close-range shots, GSR clings to the victim.) IR photographs can reveal GSR. The Griess Test may also reveal GSR.

Testing for GSR NOT foolproof. - Being near gun can leave GSR - It fades away rapidly - Can easily be washed off - Nitrates can give a false + (fertilizer, tobacco, cosmetics, urine)

Determining Distance The ME can estimate the distance from a single bullet by looking at the entry wound -If muzzle was > 2ft, it’s a small hole with an abrasion collar (blue-black halo) -If muzzle between 6in – 2ft, skin may be tattooed or stippled.

-If muzzle <6in, produces holes, a surrounding area of charring, and a bright red hue -If muzzle pressed against victim, lots of charring, ripping of skin, and star shaped stellate pattern

Shotgun Patterns The spread pattern of the shot determines distance. The shot spread ~1in for every 3ft it travels Pelvic Bone

How Does Clothing Effect?? The tattooing/charring is done to the clothes instead. The residue pattern found on clothing determines distance. The farther the distance, the wider the spread of the residue (up to a point). You need to do test fires to determine distance.