Firearms Evidence Analysis Midterm Exam

Firearm

an assembly of a barrel and action that fires projectile(s) via the products of combustion.

AFTE Theory of Identifiction

as it pertains to the comparison of toolmarks, enables opinions of common origin to be made when the unique surface contours of two toolmarks are in sufficient agreement.

What are the main parts of a revolver?

barrel, cylinder, trigger, frame, hammer, sights.

What techniques are used to manufacture breechfaces, and what marks do they leave behind?

broaching (parallel marks), milling (circular/arch marks), and filing/grinding (cross-hatch marks); other marks include smooth and granular.

Bullet examination process looks at the following:

bullet type, jacket type, base diameter, weight, number of lands and grooves, direction of twist (left or right), type of rifling (conventional or polygonal), land and groove diameter, and caliber determination.

Broach Cutting

consists of a rod bearing 25 to 30 hardened steel rings each slightly larger than the one before; steel rings are notches cut into them that correspond to the dimensions of the rifling; as it is pulled through the barrel all of the grooves are cut to the proper depth in one pass.

Scrape/Hook Rifling

consists of a rod slightly smaller than bore diameter with a set of one or two hardened steel scrapers that removes metal with each stroke.

Microscopic Comparison Conclusion - Unsuitable

contains no markings that would result in any other conclusions (i.e. bullet core).

Class Characteristics of Bullets

diameter at base, number of lands & grooves, direction of twist (left or right), type of rifling (conventional vs. polygonal), land and groove diameter, and caliber.

Electrochemical Machining (ECM)

dissolves metal into a desired shape; brass is the negative terminal and the workpiece is the positive terminal, then it is submerged into a solution of electrolytic salts and a current is passed through the solution; the areas where the work piece are touching the brass will dissolve.

Steps in manufacturing the barrel of a gun.

drilling, sizing, honing, rifling, lapping, profiling the barrel, and crowning the barrel.

How do you determine whether a bullet has a left or right twist?

examine it from the base.

Striated Marks

force and motion parallel to the surface being marked.

Impressed-Striated Marks

force and motion perpendicular to the surface being marked by the surface is striated.

Impressed Marks

force and motion perpendicular to the surface being marked.

What are the main parts of a pistol?

frame, slide, trigger, sights, barrel, magazine.

CT statute defines a handgun, revolver, or pistol as:

having a barrel length of under 12 inches.

Honing

helps smooth out any rough areas after reaming.

What is Forensic Science?

in open court, public; systematic study of the structure and behavior of the physical and natural world through observation and experimentation.

Turning

workpiece rotates while a cutter moves horizontally back and forth; process generally performed on a lathe, and considered an external cutting process.

Boring

workpiece rotates while the cutting tool remains stationary; used to enlarge drill holes from inside the workpiece; usually considered an internal cutting process.

Examples of "other weapons"

zip guns, homemade or unconventional weapons (i.e. cane guns, pen guns, etc.), short barrel rifles or shotguns, full auto firearms.

By who and when was the first comparison microscope invented and for what purpose?

1885, Alexander Von Inostrazeff developed an optical bridge that superimposed an image over another image so he could compare the color, structure, and luster of difference gems and minerals.

What is the most common number of lands and grooves?

6 pairs of lands and grooves.

First Forensic Application of the Comparison Microscope

Albert S. Osborn used the comparison microscope to perform a questioned document examination; observed colors and oxidation of ink.

"Land-Grooveian" Theorem

D = #(L+G)/3.14

Bullet weight is measured in (blank).

Grains

CT statutes defines a rifle as:

a barrel length must be at least 16 inches with an other length of at least 26 inches.

CT statute defines a shotgun as:

a barrel length pf at least 18 inches with an overall length of at least 26 inches.

Firearms Identification

a discipline of forensic science which has as its primary concern to determine if a bullet, cartridge case, or other ammunition component was fired by a particular firearm.

Toolmark Identification

a discipline of forensic science which has as its primary concern to determine if a toolmark was produced by a particular tool.

Filing

a finishing technique that is used to deburr freshly cut edges; usually hand worked and are shaped to easily fit in to small areas.

Revolver

a firearm that has a cylinder with multiple chambers that revolve around a central axis.

Full Automatic

a firearm that will discharge multiple cartridges with a single pull of the trigger.

Semi-Automatic

a firearm that will discharge one cartridge with a single pull of a the trigger.

Rifles

a firearm with a rifled barrel designed to be fired from the shoulder.

Shotgun

a firearm with a smooth bore that is designed to be fired from the shoulder.

Pistols

a handgun in which the chamber is integral to the barrel; types include semi-automatic and full-automatic/machine pistol.

Swaging

a metal working piece that uses dies to create the desired shape; a type of forging process because it applies compressive force to the metal workpiece.

Metal Injection Molding (MIM)

a process by which parts are formed by combining finely ground metallic powder, a primary binder, and a secondary binder; materials are mixed and extruded into long thin wires which are fed into a press to form the shape of the part.

Drilling

a process in which a tool creates a hole in the stock and leaves parallel marks running perpendicular to the long axis of the hole.

Electrochemical Rifling

a process that uses electricity and the flow of current to etch into the barrel the impression of the rifling.

Lead Lapping

a process that uses lead plugs impregnated with a lapping compound to smooth out the interior of the bore.

Hammer Forging

a swaging technique where the bored barrel blank is placed over a short mandrel that bears the negative of the rifling impress; as the barrel blank advances over the mandrel, hammers pound the outside of the barrel and impart rifling impressions to the interior of the barrel.

Sizing the Bore

involves reaming (increases the inside diameter of the barrel to the proper caliber) and broaching the inside diameter.

AFTE theory of identification states agreement is sufficient when:

it exceeds toolmarks made by different tools, and is consistent with agreement of toolmarks made by the same tool.

What techniques are used to manufacture firing pins, and what marks do they leave behind?

lathe turned (circular marks within the impression), metal injection molding (a line in the middle of the impression), stamped, and milled.

Reaming

metal workpiece used for smoothing out a drill hole and sizing them to the final dimension.

Extractors and ejectors are manufactured by which techniques?

milled, metal injection molding, and stamped.

Basis for Uniqueness

not every piece of metal is the same (i.e. arrangement of atoms in the crystal structure, impurities and additives), tools used to form the metal are imperfect, and the uniqueness can change over time with wear and use.

Microscopic Comparison Conclusion - Elimination

the two items were not fired by the same firearm due to a difference in individual and class characteristics.

Firing Pin Drag

occurs when the firing pin is still outside of the aperture when the firearm begins to extract and eject the cartridge casing.

Firing Pin Aperture Shear

occurs when the primer is scraped along the aperture (hole) for the firing pin in the breechface.

Bullet base descriptions

open/jacketed and flat/concaved

Firing Pin Shapes

rectangular, hemispherical (circular), elliptical.

Abrasive Machining

refers to all types of machining processes that use rough materials to finish or refine metal parts, including honing, lapping, lead lapping, grinding, and sanding.

Types of rifling used in barrel manufacturing.

scrape/hook, broach cutting, button/swaging, hammer forging, electrochemical rifling.

Electro-Discharge Machining (EDM)

sends an arc of electricity from an electrode (cutter) to the workpiece - the arc of electricity creates temperatures of over 12,000 degrees C and vaporizes the metal; creates small & accurate cuts.

Conventional/Standard Barrel Shape

sharp, well-defined shoulders of groove impression.

Types of Cannelures

smooth, knurled, crimping

Polygonal Barrel Shape

smooth, rounded shoulders of the groove impression.

The opinion is (blank); however, it is based on (blank) criteria.

subjective; objective

Chamber

the part of the firearm that contains the cartridge before it is fired located at the end of the barrel (pistol) or part of the cylinder (revolver); machined using a reamer.

Crowning the Barrel

the process of recessing the rifling at the muzzle.

Microscopic Comparison Conclusion - Inconclusive

the two items cannot be identified or eliminated from having been fired by the same firearm due to lack of detail; various levels.

Microscopic Comparison Conclusion - Identification

the two items were fired from by the same firearms based on sufficient agreement of individual characteristics.

Profiling the Barrel

turn the outside diameter of the now rifled bar stock to the proper outside dimensions.

Comparison Microscope

two microscopes connected to an optical bridge which allows the viewer to observe two objects simultaneously with the same degree of magnification.

Button/Swaging

uses a plug made of extremely hard material bearing a negative impression of the rifling; the plug is hydraulically pressed through the bore of the barrel which stretches the inside diameter and imparts rifling characteristics.

End Milling

utilizes a rotating cutter that is perpendicular to the metal workpiece; used to form slots and can be used to face the outside edges of the work piece; carbide teeth rip away at the metal.

Broaching

utilizes a series of cutters that start and finish cutting in a single pass; each cutter is slightly larger than the previous; used to make a variety of parts.