Firearms & Bullets

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

What is a Gun? A gun is a normally tubular weapon or other device designed to discharge projectiles or other material

Caliber the diameter of something of circular section, especially that of the inside of a tube: a pipe of three-inch caliber. Ordnance . the diameter of the bore of a gun taken as a unit of measurement.

http://www.youtube.com/watch?v=e8HR5TxlPMc

What is IBIS? Integrated Bullet Identification System

Gauge shotguns—report the weapon's gauge (which is the number of shot pellets having the same diameter as the bore produced from one English pound (454g) of lead)

Types of Guns Firearms Pistols, Revolvers, Shotguns, Rifles, semi-automatic

http://www.youtube.com/watch?v=RBifOqXTU4o

Types of Barrels Smooth Barrel: Hollow tube Firearms Types of Barrels Smooth Barrel: Hollow tube 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) Firearms Rifled Barrel Lands Inside of barrel has hills (lands) and valleys (grooves) Lands and grooves twist down length of gun Grooves

Rifled Barrel Inside of barrel has hills (lands) and valleys (grooves) Firearms Rifled Barrel Inside of barrel has hills (lands) and valleys (grooves) Lands and grooves twist down length of gun

Rifled Barrel (manufacturing) Firearms Rifled Barrel (manufacturing) Begin with a hollow tube Lands and grooves made by forcing cutting tools through the tube.

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

Handgun Designed to be held and fired with one hand Firearms Revolver Semiautomatic pistol

Revolver Hammer pulled back manually or when trigger is pulled Firearms Revolver Hammer Bullets Hammer pulled back manually or when trigger is pulled Hammer hits back of bullet casing; causes explosion Barrel rifled Trigger

Semiautomatic pistol Hammer strikes bullet case when trigger pulled Firearms Semiautomatic pistol Bullets Hammer Hammer strikes bullet case when trigger pulled Bullet casing automatically ejected. Barrel rifled Trigger

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

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

Ammunition (Bullets) Firearms > Ammunition Bullet Cartridge Propellant (inside cartridge) Primer

Ammunition (Types of Bullets) Firearms > Ammunition Ammunition (Types of Bullets) Lead or Lead Alloy: softer bullet; scraped up by the lands and grooves of barrel 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

Ammunition (Types of Bullets) Firearms > Ammunition Ammunition (Types of Bullets) Full Metal Jacket: lead core completely covered with brass

Fires many small round pellets Firearms > Ammunition Shotgun Shell Shot (pellets) Cartridge Wad (plastic) Propellant (inside cartridge) Primer Fires many small round pellets

Primer When hit with hammer, primer creates a small explosion 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

Collection of Firearms evidence 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

Collection of Firearms evidence 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

Analysis of Firearms evidence Firearms > Analysis Analysis of Firearms evidence Class Characteristics of bullets: Caliber (size) # of lands/grooves Direction of rifling twist Degree of twist Width of lands/grooves

Analysis of Firearms evidence 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

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

Comparison microscope is used to compare striations on bullets. Firearms > Analysis Comparison microscope is used to compare striations on bullets.

Comparison of Striations Firearms > Analysis Comparison of Striations Striations – tiny scratches on bullet that can be specific to a single gun.

Conclusions of analysis 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.

Range-of-fire estimation 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

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

Conclusions of analysis 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.

Conclusions of analysis 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.

Conclusions of analysis 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.