Firearms and Toolmarks

Firearms Summary

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

Firearms

Types of Guns

Firearms

Types of Barrels Smooth Barrel:

Hollow tube

Rifled Barrel:

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

Firearms

Rifled Barrel

 Inside of barrel has hills (lands) and valleys (grooves)  Lands and grooves twist down length of gun Lands Grooves

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  Barrel

rifled

Hammer Bullets Trigger

Firearms

Semiautomatic pistol

 Hammer strikes bullet case when trigger pulled  Bullet casing automatically ejected.  Barrel

rifled

Hammer Bullets Trigger

Firearms

Rifle

 Hammer strikes bullet case when trigger pulled  Bullet casing manually or automatically ejected.  Barrel rifled; fires bullet Trigger Hammer

Firearms

Shotgun

 Hammer strikes shotshell when trigger pulled  Bullet casing manually or automatically ejected.  Barrel smooth; fires shot or slug Trigger Projectile

Firearms > Ammunition

Ammunition (Bullets)

Bullet Cartridge Propellant (inside cartridge) Primer

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) Cartridge Wad (plastic) Propellant (inside cartridge) Primer Fires many small round pellets

Firearms > Ammunition

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 > Collection

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 > Analysis

Analysis of Firearms evidence

Class Characteristics of bullets:   

Caliber

(size)  Degree of twist # of lands/grooves  Direction of rifling twist Width of lands/grooves

Firearms > Analysis

Analysis of Firearms evidence

Class Characteristics of cartridges   

Caliber

(size)  Extractor/ejector sizes Firing pin location Firing pin size/shape  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

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

Tool Marks

Tools 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

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

Firearms > Analysis

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.