Firearms, Ballistics, Tool Marks and Arson Module Overview

Firearms and tools are manufactured and used in such a way that subtle variations can exist. These subtle differences enable investigators to link weapons and tools to a particular crime and sometimes even a suspect. Investigators can also study the trajectory of a projectile to gain more insight into the way in which the crime occurred. While arson fires and explosives are both destructive, they can still leave behind valuable evidence. This evidence can be used to reconstruct the crime as well as trace and connect materials of evidence to the suspect. Tool marks are often left in crimes such as burglary when a suspect attempts to gain entry into a secured location. These tool marks can indicate not only the type of tool used, but also a specific weapon if it has surface irregularities that can be matched to the observed tool markings.

Essential Questions

1. How can a weapon be linked to a crime or individual?
2. How are tool marks analyzed?
3. What do investigators look for to determine if a fire was accidental or arson?
4. How are explosives classified?

Key Terms

Ballistics - The study of what happens to moving projectiles in the barrel and in flight--their trajectory, force, impact and penetration.
Barrel - The metal tube of a firearm made from iron or steel, through which the bullet or shot charge passes when the firearm is fired.
Bullet - A single projectile fired from a firearm.
Caliber - The diameter of the bore of a rifle before the rifling grooves are cut.
Cartridge - A case, usually made of brass or copper, containing the power charge, the primer and the bullet. It is also known as a shell casing.
Firearm - A weapon, especially a pistol or rifle, capable of firing a projectile and using an explosive charge as a propellant.
Fully Automatic Weapon - Continuously fires rounds while the trigger is pressed and held.
Gunshot Residue - Gunshot residue consisting of burned and unburned powder, vaporized, and patriculate lead, primer resides of lead, barium, and antimony.
Lands - Raised areas between two groves 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.
Muzzle - The forward end of a barrel.
Pistol - A firearm designed to be held and fired with one hand.
Revolver - A pistol having a revolving cylinder with several cartridge chambers that may be fired in succession.
Rifle - A shoulder firearm with a rifled barrel designed to fire one projectile at a time.
Rifling - Spiral grooves cut into the inside barrel surface to cause a bullet to spin, thereby stabilizing it. The cut-away portions of the rifling are called grooves and the uncut portion are called lands.
Semiautomatic Weapon - Fires one round with each individual trigger pull.
Trajectory - The path a bullet travels from muzzle to impact.
Tool Marks - Any impression, scratch, or abrasion made when contact occurs between a tool and an object.
Abrasion Marks - Made when one surface slides across another.
Bore - The tunnel down the barrel of a firearm through which the projectiles travel.
Gauge - Measurement of shotgun bores derived from the number of bore-sized balls of lead to the pound. For example, 12 balls which fit the bore of a 12-gauge shotgun weigh one pound.
Breech - Portion of the gun that contains the firing mechanism.
Greiss Test - A chemical test used to develop patterns of gunpowder residues around bullet holes.
Accelerant - Any material used to start or sustain a fire.
Combustion - Rapid combination of oxygen with another substance, accompanied by light and heat.
Flash Point - Minimum temperature at which a liquid fuel produces enough vapor to burn.
Oxidation - Combination of oxygen with another substance to produce new substances.
Spontaneous Combustion - A fire caused by natural heat produced process in the presence of oxygen and fuel.
Shotgun - A firearm with a smoothbore designed to fire small pellets called shot or rifled slugs.
Magazine - The part of a repeating firearm which holds the cartridges or shells in position ready to be loaded one at a time into the chamber.
Firing Pin - The part of the breech mechanism which strikes the primer of the cartridge.
Cutting Marks - Produced along the edge as a surface is cut.
Indentation Marks - Made when a tool is pressed against a softer surface.
Integrated Ballistic Identification System (IBIS) - Is used to associate evidence in previously unsolved crimes. IBIS is a highly technical, computerized image analysis system that records striated images from bullets and cartridge cases and compares them to a national, and growing international, database of images.
Detonation - The sudden and violent release of mechanical, chemical or nuclear energy from a confined space which creates a shock wave that travels at supersonic speeds. It is also called an explosion.
Deflagration - The extremely rapid burning of a material. This is much much faster than normal combustion, but slower than detonation

[CC BY 4.0] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION