Forensic Science 1-Firearms and Tool Marks-University of Southern Mississippi

Firearms Identification

Discipline focused on determining whether a bullet or cartridge case was fired by a particular weapon.

AFTE

Association of Firearm and Tool Mark Examiners; offers certification for examiners.

Single-Shot Pistol

Fires one round at a time; requires manual loading for each shot.

Revolver

Several chambers in a revolving cylinder (swing-out, break-top, or solid frame).

Semi-Automatic Pistol

Uses a removable magazine; fires one shot per trigger pull; auto-cycles the next round.

Rifle

Long gun with rifled barrel (lands/grooves); bullet is impressed with rifling.

Shotgun

Smooth barrel; fires shot pellets; gauge measures bore size (higher gauge = smaller bore).

Choke (Shotgun)

Barrel narrowing that tightens shot spread.

Firing Sequence (Basics)

Trigger → firing pin hits primer → powder ignites → gases propel bullet → case pushed back on breechface.

Rifling

Spiral grooves in bore that impart spin to bullet.

Lands & Grooves

Lands = raised portions; grooves = cut portions forming rifling.

Caliber

Barrel diameter measured between opposite lands.

Class Characteristics (Firearms)

Manufacturer-set features like number/width of lands & grooves, direction of twist, caliber.

Striations (Individual Characteristics)

Fine random lines from machining/wear that uniquely mark bullets and cases.

Initial Bullet Comparison Points

Number of lands/grooves and direction of twist (class characteristics).

Individualization (Bullets)

Match of microscopic striations under a comparison microscope.

Cartridge Case Individualization

Matching firing pin impressions, breechface marks, extractor and ejector marks.

Breechface Mark

Impression pattern transferred to case head during rearward thrust.

Firing Pin Impression

Shape/marks from pin striking primer cup.

Extractor / Ejector Marks

Tooling marks from mechanisms withdrawing and expelling the case.

NIBIN

National Integrated Ballistics Information Network; database of bullet and casing images to link shootings.

Examiner's Role with NIBIN

Database suggests candidates; final ID is by traditional microscopic comparison.

Purpose of Distance Determination

Estimate muzzle-to-target distance via residues/patterns around bullet hole.

With Suspect Weapon

Compare test-fire patterns (shape/density) to evidence on clothing/skin from varying distances.

Contact to <1 inch

Star-shaped tearing, smoke/lead deposit rim, scorching; synthetics may melt.

12-18 inches or less

Soot (vaporous lead) and partially burned powder present.

25-36 inches

Partially burned powder; no soot.

3 feet+

No powder/soot; possible bullet wipe ring (carbon, dirt, lubricant, residues).

Shotgun Distance (No Weapon)

Estimate by pellet spread—greater distance → wider spread.

GSR Backspatter

Unburned/partially burned powder and primer residues blown back to shooter's hands/garments.

Hand Sampling for GSR

Adhesive stubs on thumb web, back of hand, and palm; SEM/EDS for barium, antimony, Pb.

Purpose

Recover obliterated firearm serial numbers.

Method

Chemical etching exploits strained metal in stamped zone to reveal digits.

Safe Handling of Firearms

Hold by trigger guard edge/checkered grip; unload safely; prevent accidental discharge.

Revolver Documentation

Record chamber positions and which chambers contain cartridges.

Marking & Chain of Custody

Tag firearm (e.g., on trigger guard) and maintain full chain.

Packaging Bullets/Cartridges

Avoid damaging striations; wrap bullets in tissue; place in pillbox/envelope; label properly.

Tool Mark

Impression/cut/gouge/abrasion made when a tool contacts another obje

Class vs. Individual (Tool Marks)

Class: size/shape/type; Individual: microscopic imperfections from manufacture/use/wear.

Comparison Method (Tool Marks)

Comparison microscope between test impressions and crime-scene marks.

Casting Tool Marks

Silicone putty casts when objects cannot be collected whole.

Do NOT Fit Tool to Mark

Contact can alter evidence and compromise integrity; never attempt a direct fit at the scene.

AFTE

Association of Firearm and Tool Mark Examiners.

NIBIN

National Integrated Ballistics Information Network.

Class Characteristics (Firearms)

Caliber, land/groove count/width, twist direction.

Individual Characteristics (Firearms/Tools)

Unique microscopic striations/impressions from manufacture and wear.