Forensic Science Unit 4 Vocab

differences between rifles and shotguns

Barrel and ammunition.

Double action

A single pull of the trigger cocks and releases the hammer.

Cartridge

A unit of unfired ammunition.

Manufacturing of firearms

All surfaces of firearms are machined during manufacturing process; machining process leaves marks that are specific to that particular method; marks can be rough enough to be seen with the naked eye or smoother, requiring the aid of a microscope.

Rifles

Ammunition: bullet; barrel with rifling: bullet is impressed with lands and grooves during firing.

Shotguns

Ammunition: shell; barrel: smooth without grooves and lands; narrowing toward muzzle (called the choke) can concentrate shot when fired; diameter expressed by the term gauge: the higher the gauge, the smaller the barrel's diameter; single or double barrel (arranged horizontally or vertically).

reloading mechanisms (long guns)

Bolt action: firearm manually loaded by reciprocation and engagement of a bolt Lever Action: breech mechanism cycled by external lever below the receiver Slide Action: a movable forearm that is manually actuated in motion parallel to the barrel by the shooter (also known as pump action)

4 class characteristics of a barrel

Caliber: diameter of the gun barrel, measured between opposite lands. Number of lands and grooves. Direction of twist. Width of the lands and grooves.

Bottleneck cartridge (rifle ammunition)

Cartridge case has main diameter and distinct angular shoulder stepping down to a smaller diameter at the neck position. Larger volume in the case to hold more gun powder; helps rifle cartridges achieve higher velocities than handgun cartridges.

Shot shell (shotgun ammunition)

Cartridge containing many projectiles that are designed to be fired simultaneously; cartridge body may be metal, plastic, or paper. Shot: typically lead or steel. Wadding: plastic cup that holds the shot.

Greiss test

Chemical tests conducted to detect gunpowder residue; may be needed to detect gunpowder residues that not visible.

Cannelure

Circumferential groove (knurled or plain appearance) in bullet or cartridge case on revolver and rifle bullets. Reason: has waxy lubricant to keep barrel clean and surface that holds bullet in place.

Shell

Contains numerous ball-shaped projectiles called shot: small lead balls or pellets that are not impressed with any characteristic markings that can be related back to the weapon.

Broach cutting

Cutting all grooves in one pass with a cutter known as a broach. Toll has series of cutting edges of slightly increasing height used to cut the spiral grooves in a barrel.

Wadcutter

Cylindrical bullet; used for target shooting; sharp shouldered nose intended to cut paper cleanly to facilitate easy and accurate scoring.

Lead round nose

Elongated projectile made of a lead alloy with a rounded nose; used for target shooting.

Striations (individual characteristics)

Fine lines found in the interior of the barrel; marks produced by random imperfections or irregularities; produced incidentally by manufacturing process; caused by use, corrosion, or damage; form the individual characteristics of the barrel; inner surface of barrel leaves is striation markings on a bullet passing through it.

Bullet examination

First: differences in class characteristics serve to eliminate the possibility that both bullets traveled through the same barrel. Second: Individualization (no two barrels will have identical striation markings)

Manufacturing firearm barrels

Gun barrel is produced from a solid bar of steel hollowed out by drilling; drill marks are left on barrel's inner surface; class characteristics: rifling; individual characteristics: striations, randomly irregular, unique to each barrel.

Types of Firearms

Handguns; long guns: rifles and shotguns

Rifling

Imparts a spin on the bullet as it travels down the barrel; bullet continues to spin as it exits the barrel and flies through the air; rotation of the bullet increases its stability in flight; grooves are cut in a spiral pattern; between grooves are raised areas called lands.

Gun barrel markings

Inner surface of barrel of a gun leaves its class characteristic markings on a bullet passing through it; these markings are peculiar to each type of gun.

Jacketed hollow point

Jacket (metal) surrounding the lower portion; nose of the bullet has a hollow point; favors expansion or "mushrooming" upon contact with object; increases the surface area of the bullet; prevents over penetration; used for hunting and self-defense.

Jacketed soft point

Jacket (metal) surrounding the lower portion; soft lead core exposed at the nose; allow for expansion of bullet upon impact with an object; typically used for hunting.

Flat surfaces of firearm parts

Machined on a milling machine; cutting tool of milling machine (endmill) can cut with either the side or the end.

Cut rifling

Made by a broach cutter or button; rifling with sharp edges.

Full metal jacket

Metal encloses entire bullet (usual exception of the base); most common use is target shooting.

Comparison microscope

Most important tool to firearm examiners; two bullets observed and compared simultaneously within the same field of view; for a match: class characteristics must coincide (lands and grooves of the test and evidence bullet must have identical widths), AND longitudinal striations on each must coincide.

Shot 12 to 18 inches or less

No stellate tear pattern; halo of vaporous lead (smoke) deposited around a bullet hole.

Headstamp

Numerals, letters, and symbols (or combination) that are stamped into the base of a cartridge case or shot shell; used to identify manufacturer, caliber, gauge, or give additional information.

Bullet

The projectile typically made of lead and can be "jacketed" or covered with copper.

Button rifling

Pressing all the grooves at once onto the barrel with a tool known as a button; hardened metal plug with a rifled cross section configuration; it is pushed or pulled through a drilled and reamed barrel to form the spiral grooves to the desired depth and twist.

Gun powder

Propellant; when gun powder burns it produces large amounts of gas, and the expansion of this gas pushes the bullet through the barrel.

Single action

Requires manual cocking of the hammer before sufficient pressure on the trigger releases the firing mechanism.

Long guns

Rifles and shotguns; designed to be fired while resting on shoulder; single-shot vs repeating; semiautomatic vs automatic.

SEM-EDX

Scanning electron microscopy-energy dispersive X-ray spectroscopy Result characteristics of the element from which it was emitted; non-destructive analysis; used to determine amount of specific metal in bullet fragments.

Shot at distance up to 25-36 inches

Scattered specks of unburned and partially burned powder grains without any accompanying soot.

Primer

Separate component placed in the cartridge; contains shock-sensitive explosive that produces sparks when hit by a firing pin; these sparks ignite the gun powder causing it to burn.

Revolvers

Several firing chambers within a revolving cylinder; several chambers arranged to rotate around an axis.

Handguns

Single-shot pistols: fire only one round at a time Revolvers: several firing chambers within a revolving cylinder; rounds are discharged successively by the same firing mechanism. Semi-automatic pistols: removable magazine with multiple ammunition that fire one shot per trigger pull.

Boat tail

Tapered or a truncated conical base; provides increased stability throughout its flight.

Semi-wadcutter

Truncated nose and sharp shoulder; used for target shooting; sharp shoulder on the bullet produces a clean round hole in paper.

Chemical etching

This method can be used to restore a serial number on a firearm that has been removed or obliterated by grinding, rifling, or punching. Metal crystals in the stamped zone are placed under a permanent strain that extends beneath the original numbers.

Polygonal rifling

This type of rifling is produced with hammer forging; rifling with rounded profile.

Rimfire (rifle ammunition)

Type of ammunition that has priming compound around circumference of the base; does not have a primer as a separate component; when firing pin crushes anywhere along the rim, the primer is ignited.

Cartridge case

Typically made of brass and holds all the parts of the cartridge together.

Shot more than 3 feet away

Usually no deposit of any powder residue; only visual indication is a dark ring around the hole known as a bullet wipe.

Contact shot

Weapon held in contact with or less than one inch from the target; star-shaped (stellate) tear pattern around the bullet hole entrance; surrounded by a rim of a smokeless deposit vaporous bullet hole.

Round parts of firearm

When cutting with the end, surface has circular or arch marks; machined with a lathe, which spins the work piece around an axis.

When bullets hit objects

When this happens, the bullets may break apart or become deformed; when they break apart from contact, the pieces are called fragments; even tiny fragments have forensic value.