Forensics Chapter 15- Firearms, Toolmarks, and Impressions

caliber

the diameter of the bore of a firearm or the cartridge that is fires, expressed in hundredths of an inch

bore

interior diameter of a gun barrel

gauge

interior diameter of a shotgun barrel as determined by the number of lead balls of a size of exactly fitting the barrel that are required to make one pound

shot

lead spheres in a shotgun shell

slug

single shot or bullet in a shotgun shell

grains

basic units of weight in the British system, originally based on the weight of a grain of barley (1 gram\= 15 grains)

rifled

bore of a gun barrel that has been machined to from a grooved spiral, much like a helix

lands

raised areas between two grooves in the rifling of a gun barrel that impart grooves on the bullet

grooves

spiral cuts into the bore of a barrel that give the bullet its spin or rotation as it moves down the barrel

striae

parallel sets of scratches on a bullet caused by unique markings in the bore of a rifled weapon

cannelures

grooves around a bullet that provide a means of crimping the cartridge case to the bullet

head stamp

numerals, letters, and symbols on the base of a cartridge showing the manufacture, caliber, and code

breech

portion of a gun that contains the firing mechanism

extractor

hooked or crescent- shaped part attached to the breechblock which withdraws the spent casing from the chamber when the breechblock separates from the barrel after firing

ejector

the part on the firearm whose function is to throw a spent casing from the gun after firing

magazine

a container that holds cartridges under spring pressure to fed into the gun's chamber; also a clip

chamber

part of the firearm that contains a cartridge for firing

Greiss test

color test for the presence of nitrite in the gunpowder residue

GSR

gunshot residue consisting of burned and unburned powder, vaporized and particulate lead, primer residues of lead, barium, and antimony

sodium rhodizonate test

color test for the presence of lead in gunshot residue

buffer

a solution containing either a weak acid and its salt or a weak base and its salt, which is resistant to changed to changes in pH

outsoles

outer souls of shoes that are the bottom of the shoe, in contact with the ground

gait

manner of walking or moving on foot, said to be as individualized as your personality; can also be determined if multiple footprints are visible.

wheelbase

distance from the center of front wheel hub to center or rear wheel

stance

distance from the centerline of the right tire to the centerline of the left tire

Types of Firearms

Handguns
Rifles
Revolvers
Semi-Automatics
Snubnose Revolvers
Assault Weapons
Saturday Night Specials

Handguns

compact concealable weapons that can be operated one-handed.

Revolvers

small guns with a revolving cylinder that turns after the gun is fired and puts another bullet into position to be fired next. Do not eject shells, and must be reloaded by hand.

Semi-Automatic

Small guns with re-loadable clips that are able to fire repeatedly but require release and another pull of the trigger for each successive shot. Do eject shells, and use re-loadable magazines.

Rifles

Larger guns that are much harder to conceal and take two hands to operate. Have a long barrel with a rifled bore and a stock,and are designed to be fired from the shoulder. Generally fire more accurate and powerful cartridges and fire shells packed with a number of smaller projectiles referred to as shot in a spread patters.

How are modern barrels made?

by drilling out a solid piece of steel.

How can barrels be used as evidence?

The drilling leaves characteristic drill marks and grooves that can identify a weapons manufacturer or individualize a specific weapon.

Bullet

A small metal projectile, part of a cartridge, for firing from firearms.

What are bullets made of?

lead jacketed in a harder metal like brass, copper, or steel.

Parts of Cartiridges

a case, primer, powder, and bullet.

What goes into the ballistic fingerprinting of bullets?

Caliber - Projectile diameter
Shape - Jacket type, general shape, hollowness etc.
Weight - Measured in grains (1 gram \= 15 grains)
Land and groove impressions - Number, width, twist
Striae - Microscopic markings left by the individual wear of a firearms barrel.

What goes into the ballistic fingerprinting of cartridges?

Dimensions - Height, width, diameter.
Head Stamp - Manufacturers mark on the base.
Rimfire or Centerfire - Primer type.
Unique Markings - The firing pin, breech, ejector and extractor, magazine, and chamber of a firearm all leave unique microscopic markings on a cartridge.

What are cases normally made out of?

brass

What does the propellant normally contain?

nitrocellulose and nitroglycerin.

What does the primer usually contain?

lead styphnate, barium nitrate, and antimony sulfide.

What are the two tests for ballistics?

Greiss Test and the Sodium Rhodizonate Test

What is the Greiss test usually used for?

normally used to determine the distance a bullet was fired from.

What are tools often used for in crimes?

weapons, or to gain entrance to an area during crimes.

What is considered class evidence in toolmarks?

size and shape of the tool

What is considered individual evidence in toolmarks?

individual pattern of wear a tool develops through use

How are toolmarks recorded and tested?

1. Photographed or drawn.
2. If the marked item is small it is logged as evidence. If not, an impression of the marked area is made.
3. The microscopic pattern a tool leaves is often the determining factor in an investigation.

What kind of evidence are impressions?

physical evidence and class evidence
Impressions can be individualized in some cases.

What can impressions tell us?

the identity of a suspect,victim,vehicle, etc. sequence of events, and determine fault in some accident cases.

How are shoeprints often left at a crime scene?

by transfer. - show the gait of a person

How can shoeprints be collected?

photography, oblique photography or casting.

Tire Treads

often leave imprints or transfer rubber to surfaces.

How can tire treads be collected?

through photography, oblique photography or casting.

What can tire marks be used for as evidence?

help prove where a car has been, reconstruct car accidents, and even be individualized if a unique wear pattern is present.

When are bite marks commonly used as evidence?

assault cases as well as when domestic abuse occurs.

How can bite marks be collected?

by photography, dental casting, and can also provide valuable DNA evidence in some cases. Normally the victim's wound will be photographed and then compared to a casting of the suspects teeth or dental records if available.

How can serial numbers be restored?

Serial numbers are usually stamped onto the metal of an item during manufacturing. This causes a different crystal structure in the area below each number, and can be revealed if exposed to an etching solution like Acidic Cuprous Chloride (HCl -CuCl2).