Forensics Firearms

Cartridge: unti of ammunition consisting of the cartridge case, primer, and propellant with or without one or more projectiles

Cartridge designation: an identifier created by the manufacturere, which often includes a numerical value to indicate the approximate diameter of the bullet and the name of the cartridge manufacturer

Action: the part of the firearm that loads, fires, and ejects a cartridge

1. Single-action revolver
2. Lever action
3. Pump action
4. Boly action
5. Semi-automatic

Caliber: diameter of a bullet, often expressed in hundredths of an inch (eg. 22-caliber bullet has a diameter of 0.22in)

Class characteristic

1. When the trigger is pulled, the hammer or firing pin impacts the base of the cartridge and detonates the primer
2. Detonation of the primer forces the hot gases and particuates into the propellant powder, causing the ignition of the propellant
3. Primers are pressure-sensitive materials

1. Ammunition is loaded into a magazine, which is inserted through the underside of the grip
2. Semi-automatic pistols can shoot multiple bullets by consecutive trigger pulls

1. Require longer bullets for more gunpowder
2. Usually shot for longer distances

Gauge: the diameter of the barrel on the shotgun in terms of the number of lead balls it would ake to weight one pound (12 gauge in a diameter of a lead ball weighing 1/12 of a pound)

Rifling: the forming of grooves in a spiral nature along the length of the barrel of a firearm

The rifling pattern in the barrel imparts a "negative" imprint onto the softer bullet, resulting in the fired bullet being marked with a pattern of lands and gooves complementary to that in the barrel

Toolmark: a mark formed when one object (harder) forms contact with, and subsequently leaves an impression on another object (softer)

1. Collection of bullets and cases, documentation of characteristics
2. Examination of questioned firearms recovered from scene or from suspects
3. Identification of firearms class characteristics
4. Test firing: comparison of test fire bullets/casings with those from bullets/casings recovered at the scene

Comparison microscopy - two microscopes connected by an optical bridge that allows for the side-by-side comparison of two objects at the same magnification, focus, etc.

1. A tank is filled with water or cotton
2. The water/cotton slows down the bullets with minimal compromise to surface features

1. To determine the rifling pattern in the gun that fired a recovered bullet, count the number of groove and land impressions around the circumference of the bullet
2. Looking from the back end of the bullet, the twist direction is given by the direction the impressions tilt away from the axis (either left or right)
3. Measurements of the rifling impressions on the bullet

1. Cartridge length
2. Breech marks
3. Firing pin impressions
4. Extractor marks
5. Ejector marks

1. Striated action marks occur on cartridge cases that have passed through the action of an auto-loading or repeating firearm - considered "scraping" in nature
2. Produced on cartridge cases by contact with a number of different areas within the action of the firearm
3. Includes chamber marks, shear marks, firing pin drag marks, extractor marks, and ejector marks

1. Firing pin impressions are created through the striking of the primer (center for center fire, rim for rim fire)
2. If the nose of the firing pin has manufacturing imperfections or damage, these potentially unique characteristics can be impressed into the metal of the primer or rim of the cartridge case

1. Marks produced by the random imperfections or irregularities of tool surfaces (produced incidental to manufacture and/or cause by use, corrosion, or damage) - unique to that tool and distinguishes it from all other tools
2. The individual characteristics are transferred by the firearm components (barrel, firing pin, extractor, ejector) to the bullet or cartridge case

1. The high pressure formed when the firearm is discharged forces the bullet from the cartridge case and down the barrel at very high velocities
2. Generates a pressure towards the shooter known as recoild, driving the cartridge backward and against the breech face
3. Recoil is largely caused by the cartridge case moving rearward as an opposite reaction to the pressures generated to force the bullet down the barrel

1. Integrated Ballistic Identification System (IBIS)
2. A system wherein participating agencies collect images of scene bullets and cartridge caings, and upload them to a database linking participating jurisdictions

1. No set number of characteristics to establish commonality of source
2. Bullets/projectiles are often very badly damaged and unsuitable for analysis

1. Scientific Working Group for Firearms and Toolmarks (SWGGUN)
2. Establish a set of standards for what is ultimately conclusive for a statement of identificationl and standards for practice (incl. evidence documentation)

Association of Firearm and Toolmark Examiners (AFTE)

"Signifcant disagreement of discernible class characteristics and/or individual characteristics."

1. Condition of the working surface of the tool and/or substrate
2. Material composition of the working surface of the tool and/or substrate
3. Time of event to time of analysis factors
4. History of the tool
5. Numer of items
6. Consistency and reproducibility of the individual characteristics

Identifications - agreement of a combination of individual characteristics and all discernible class characteristics, where the extend of agreement exceeds that which can occur in the comparison of toolmarks made by different tools and is consistent with the agreement demonstrated by toolmarks known to have been produced by the same tool

1. Agreement of all discernible class characteristics without agreement or disagreement of individual characteristics due to an absence, insufficiency, or lack of reproducibility
2. Agreement of all discernible class characteritics and disagreement of individual characteristics, but insufficient for an elimination