Firearms and Toolmarks

Firearms Summary

Guns (types and manufacturing) Ammunition Firing a gun Collecting Evidence Analyzing Evidence

Types of Guns

Firearms

Smooth Barrel: Hollow tube

Firearms

Types of Barrels

Rifled Barrel: Barrel of gun can be rifled to put spin on bullet; increases accuracy

Rifled Barrel

Inside of barrel has hills (lands) and valleys (grooves)

Lands

Grooves

Lands and grooves twist down length of gun

Firearms

Rifled Barrel

Inside of barrel has hills (lands) and valleys (grooves)

Lands and grooves twist down length of gun

Firearms

Rifled Barrel (manufacturing)

Begin with a hollow tube Lands and grooves made by forcing cutting

tools through the tube.

Firearms

Rifled Barrel (manufacturing)

The minor differences in manufacturing leads to differences in the scratches (striations) on the fired bullet.

Firearms

Handgun

Designed to be held and fired with one hand

Revolver Semiautomatic pistol

Firearms

Revolver

Hammer pulled back manually or when trigger is pulled

Hammer hits back of bullet casing; causes explosion

Hammer Bullets

Trigger

Barrel rifled

Firearms

Semiautomatic pistol

Bullet casing automatically ejected.

Hammer Bullets

Trigger

Hammer strikes bullet case when trigger pulled

Barrel rifled

Firearms

Rifle

Hammer strikes bullet case when trigger pulled

Bullet casing manually or automatically ejected.

Bullets

Trigger

Bullets

Hammer

Barrel rifled; fires bullet

Firearms

Shotgun

Hammer strikes shotshell when trigger pulled

Bullet casing manually or automatically ejected.

Projectile

Trigger

Barrel smooth; fires shot or slug

Firearms

Ammunition

(Bullets)

Bullet

Cartridge

Primer

Propellant (inside cartridge)

Firearms > Ammunition

Ammunition (Types of Bullets)

1. Lead or Lead Alloy: softer bullet; scraped up by the lands and grooves of barrel

2. Semijacketed bullet: brass jacket covers sides of lead bullet, except for tip

Bullet expands on impact Jacket not scraped up as much Jacket can be shed after hitting target

Firearms > Ammunition

Ammunition (Types of Bullets)

3. Full Metal Jacket: lead core completely covered with brass

Firearms > Ammunition

Shotgun Shell

Shot (pellets)

Wad (plastic)

Cartridge

Primer

Propellant (inside cartridge)

Firearms > Ammunition

Fires many small round pellets

Primer

When hit with hammer, primer creates a small explosion

This explosion lights propellant creating a large explosion (propels bullet)

Firearms > Ammunition

Propellant

Modern propellants are smokeless powders composed of nitrocellulose

Some contain nitrocellulose and nitroglycerin

Firearms > Ammunition

Collection of Firearms evidence

Important marks on the outside of the bullet can be damaged during collection.

Either collect the item containing embedded bullet or…

Use rubber coated forceps to remove bullet

Firearms > Collection

Fired Bullets

Firearms > Collection

Collection of Firearms evidence

Bullets and cartridges should probably not be physically marked by investigator. Marks may damage important striations

Small evidence should be collected in sealed and marked containers Pillbox, glass vials

Firearms > Collection

Analysis of Firearms evidence

Class Characteristics of bullets:

Caliber (size) # of lands/grooves Direction of rifling

twist

Degree of twist Width of

lands/grooves

Firearms > Analysis

Analysis of Firearms evidence

Class Characteristics of cartridges

Caliber (size) Firing pin location Firing pin size/shape

Extractor/ejector sizes Relative location of

extractor/ejector

Firearms > Analysis

Analysis of Firearms evidence

If class characteristics of fired bullet and suspected gun match, a test bullet is fired for further analysis.

Firearms > Analysis

Comparison microscope is used to compare striations on bullets.

Firearms > Analysis

Comparison of Striations

Striations – tiny scratches on bullet that can be specific to a single gun.

Firearms > Analysis

Conclusions of analysis

If class characteristics and striations match, can say the bullet was likely fired by the suspect gun.

If class characteristics (and striations) do not match, the suspect gun can be excluded.

If class characteristics match but striations do not, the results are inconclusive.

Firearms > Analysis

Range-of-fire estimation

The distance between the gun and victim can be estimated by looking at gunshot residue patterns

Long distance: no GSR on the victim

Close-range: GSR on victim - spread out

Near-contact: GSR on victim - concentrated circle

Contact shot: “Blown out” entrance wound

Firearms

Tool MarksTools make specific marks when they cut or

scrape a softer surface

Compression tool mark: shows outline of tool (hammer into wood)

Sliding tool mark: parallel striations when tool slides across material (screwdriver or crowbar)

Cutting tool mark: striations when tool cuts through material (scissors)

Tool Marks

Collection of Tool Marks

Notes, sketches, photographs

If possible, the tool mark should be taken back to the lab.

If not, a cast should be made.Silicone rubber ideal material

Tool Marks

Conclusions of analysis

If class characteristics and striations match, can say the tool mark was likely made by the suspect.

If class characteristics (and striations) do not match, the suspect tool can be excluded.

If class characteristics match but striations do not, the results are inconclusive.

Firearms > Analysis