Ballistics and Firearms Bullet and Cartridge Comparisons

What is Ballistics? Ballistics, the science that deals with the motion of projectiles such as bullets, shells, rockets, and aerial bombs. three branches: Interior ballistics, Exterior ballistics and Terminal ballistics

Interior Ballistics The behavior of a projectile within a gun barrel A firearm such as a rifle, pistol, or artillery piece fires a projectile as a result of the burning of its propellant, which usually is smokeless powder. When the powder is ignited, large quantities of gases are produced. These gases force the projectile through the barrel.

Exterior Ballistics The motion of a projectile in flight When a bullet or artillery shell leaves a gun, it spins like a top because of the spiral rifling. This spinning motion gives the projectile stability of flight. While in the air, the projectile is subject to various forces, such as gravity, air resistance, wind, and drift caused by the spin.

Terminal Ballistics The effect of the projectile on its target The penetration of tank armor by armor-piercing ammunition is an important concern of terminal ballistics. Another is the damage done to the target by the blast effect and flying fragments of a shell or bomb. The study of terminal ballistics helps scientists to develop more effective weapons and to devise means of defense against enemy weapons.

Parts of a Gun 1. Gun Barrel: solid bar of steel that has been hollowed out by drilling. The barrel is the part the bullet moves through after it is fired. The muzzle is the open end of the barrel that the bullet exits the gun through.

The Gun Barrel cont. Bore = the interior of a firearm barrel Rifling = spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when it is fired – the surface of the original bore remaining between the grooves are called lands.

The Gun Barrel cont. Caliber = diameter of the bore of a rifled firearm, usually expressed in hundredths of an inch or millimeters (.22 caliber and 9mm) – may not be exact Striations = fine lines, imperfections, irregularities – to see need comparison microscope – no two rifled barrels have identical striation markings

Parts of a Gun 2. Action: The action is the part that has the mechanism that fires the bullet. My rifle picture is a bolt action. There are also lever action, semi automatic, break action single shot rifles. The top handgun is a semi automatic. The bottom is a revolver.

Parts of a Gun 3. Grip or Stock: The grip or stock is the part you hold onto. Generally it's a grip on a handgun and a stock on a long gun. 4. Trigger Guard: The piece around the trigger that protects it from getting bumped.

Parts of a Gun 5. Trigger: The part your finger pulls to make the gun fire. 6. Sights: Sights come in various styles also. Most firearms have a front sight at the muzzle end of the barrel, and a rear sight at the back end of the barrel.

Class Evidence Same number of lands and grooves Width and direction of twists FBI – General Rifling Characteristics File

Individual Characteristics Striations As a bullet passes through the barrel, its surface is impressed with the rifled markings of the barrel – test firings in either a recovery box or water tank

Considerations in Bullet Comparison Rarely encounter a perfect match all around the bullet’s periphery – Why? – grit, rust alter markings and bullets distorted on impact Striations marking on a barrel are not permanent – subject to alteration through wear

Considerations in Bullet Comparison A shotgun has a smooth barrel (so no impressed markings) Generally fires small lead balls or pellets contained in a shell (can use weight and diameter to identify) Diameter of a shotgun barrel is expressed by the term gauge (the higher the gauge number, the smaller the barrel’s diameter)

Cartridge Cases (helps with individualizing) Firing pin is impressed into the soft metal of the primer on the cartridge case – reveals distortions of the firing pin Breechblock – rear part of a firearm barrel

Cartridge Cases (helps with individualizing) Other distinctive markings Extractor – mechanism in a firearm by which a cartridge or fired case is withdrawn from the chamber Ejector – mechanism in a firearm that throws the cartridge or fired case from the firearm Magazine or clip Imperfections on the firing chamber walls

Automated Firearms Search Systems Computerized imaging technology – storage of bullet and cartridge surface characteristics FBI – DRUGFIRE ATF – IBIS (Integrated Ballistic Identification System)

Automated Firearms Search Systems The FBI’s system and the ATF’s system were competing and incompatible In 1999 joined forces – NIBIN (National Integrated Ballistic Information Network) Screening tool – final comparison must be made by a forensic examiner through traditional microscopic methods

Gunpowder Residues Distance determination – usually based on distribution of powder patterns or the spread of a shot pattern Use test comparison – fire suspect weapon and suspect ammunition at test target

Gunpowder Residues Look for recognizable characteristics around the bullet hole Weapon in contact or less than 1 inch away when discharged – bullet hole has smoke-like vaporous lead surrounding hole Loosed fiber shows scorch marks or synthetic fibers shows signs of being melted

Gunpowder Residues A weapon fired more than 3 feet from a target usually does not deposit any powder residues on the target’s surface – only visual indication that a hole was made by a bullet is a dark ring, known as bullet wipe, around the perimeter of the entrance hole Shotguns – rule of thumb – spread in pattern made by a 12-gauge shotgun increases 1 inch for each yard of distance (example 10 inch pattern = 10 yards) – but also depends on barrel length, size and quantity of pellets fired, quantity of powder charge, and the choke (degree of constriction placed at the muzzle end of the barrel)

Powder Residues on Garments Photograph using different light – infrared Chemical tests Greiss Test (picks up nitrate particles) Lead Tests

Primer Residues on the Hands Made by measuring the presence and possibly the amount of barium and antimony on the suspect’s hands Adhesive tape (can use scanning electron microscope – X-ray analyzer, this is time consuming) Cotton swab moistened with 5% nitric acid (for living people – need to do quickly – no more than 2-6 hours after the crime) Atomic Absorption Spectrophotometry

Primer Residues on the Hands Problems Major difficulty – short time primer residues remain on the hands Cannot be used to detect firings with most .22 caliber rim-fire ammunition – such ammunition may contain only barium or neither barium nor antimony in its primer composition

Collection and Preservation of Firearms Evidence Criminalistics can restore serial numbers removed or obliterated by grinding, rifling or punching (How? – because the metal crystals in the stamped zone are placed under permanent strain that extends a short distance beneath the original numbers, the serial number can be restored through chemical etching)

Collection and Preservation of Firearms Evidence A suspect firearm should never be picked up by inserting an object into its barrel because this practice may alter the striation markings on test-fired bullets Before unloading a suspect weapon, the weapon’s hammer and safety position should be recorded, as well as the location of all fired and unfired ammunition in the weapon

Collection and Preservation of Firearms Evidence Protection of class and individual markings on bullets and cartridge cases is the primary concern of the field investigator when recovering bullets and cartridge casings Use caution when removing a lodged bullet from a wall or other object – best to carefully break away surrounding support material while avoiding direct contact with the projectile Tag with initials of investigator

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