forensics quiz 2

what does firearm identification include?

1) bullet comparison

2) knowledge of weapons

3) restoration of removed serial numbers

4) gun shot residue characterization

5) what was this gun used for

how can you compare bullets?

1) each gun has its own inner surface markings that will score the bullet at the base/exterior, as the bullet leaves the inner surface of the gun leave individual surface markings

2) test fire a bullet and compare the markings made

3) if there is a match you have a gun/suspect because the way the barrel of the gun scratches the bullet is individual to the gun

what are characteristics of the barrel of a gun

1) inner surface has irregular random microscratches (individualized)

2) inner surface is impressed with a spiral, grooves are known as rifiling

what are lands

the original bore between two grooves

what is a caliber

the distance between two lands (recorded in hundreths of an inch or mm)

how can you determine the number of lands and grooves from a bullet

1) grooves come from land, lands come from grooves

2) must know caliber

3) measure groove impression

4) measure land impression

—> (caliber) / (groove diameter + land diameter) * pi

what is a metal jacket

coating on outside of projectile (bullet), usually copper

what is a wad cutter

• a bullet best for upclose

1) jacketed: coated

2) plain lead: no coating

what is a hoxie

• best for upclose

1) partial jacketed

- Ball at end up bullet increases fragmentation, can get in between bones

- low accuracy

what is a jacketed flat nose

• has more accuracy, but still mostly used for upclose

- all lead with small slits in the metal jacket

- when you hit the bullet will expand

what is a round nose

• has more accuracy, but still used for upclose

• 38 caliber

- fully jacketed, doesnt expand (mushroom) as much

what are hollow hips

• still handgun, not best accuracy

- fully jacketed (copper)

- has a hole which expands and blows up everything

what are talons

• military use ONLY, “cop killers”

• 45 caliber

- spreads open like talons of an eagle

- cuts through a bullet proof vest because of tungsten tip, penetrates steel

what are the mechanics behind pulling a trigger of a gun

1) pull trigger

2) firing pin strikes primer

3) ignites powder inside

4) powder expands

5) projectile moves out

• as bullet moves forward, casing moves backwards with equal force

• strikes breechblock marking the casing (individual characteristic)

• firing pin marks primer uniquely (unique class characteristic)

semiautomatic bullets

1) date, brand, caliber, and primer on bottom

2) has no lip at the base of the cartridge

revolver shell casings

1) date, brand, caliber, and primer on bottom

2) has a lip at the base of the cartridge

how do cops find guns?

integrated ballistic ID system (IBIS)

• contains ~800,000 images of

  • spent cartridges

  • fire bullets

how can you determine the distance of where someone shot a person?

• based on powder distribution

• closer shot - less dissipation of powder

• burn intensity - suicide has less spread of powder and higher burns

how do you locate gunshot residue on someone?

1) usually found on back/webbing of hand

• gas comes out of the back of the gun

2) most primers are lead, barium and antimony based (except 0.22)

3) swab the hand with 5% nitric acid, the metals will dissolve

• can use AAS or SEM w/ x-ray analyzer to determine

what are ghost guns

guns that have been manufactered by a criminal and can’t be located in the data base

how do investigators identify etched off serial number

1) use and etching material such as HCl

2) the stressed area will dissolve faster than an area not touched (since serial numbers are stamped deep a criminal cant get rid of it completely)

3) should be able to locate atleast most of the serial number (usually 7-9 numbers long)