Firearms ID Presentation

Polson High School

What is Firearms Identification?

  Sometimes incorrectly referred to as ballistics.•The identification of fired bullets, cartridge cases or other ammunition components as having been fired from a specific firearm.•Firearms identification is actually a form of Tool Mark Identification.

•Firearms evidence submitted to a lab's Firearms Section will typically include a firearm, fired bullets, spent cartridge cases, spent shot shells, shot, shot shell wadding, live ammunition, clothing, and most anything else.

In addition to comparing ammunition components to firearms, firearm examiners conduct other examinations that usually include the following:

•Testing firearms to determine if they function properly.

•Determine caliber and manufacturer of ammunition components.  Including the examination of various shotshell components

•Determine the manufacturer or manufacturers of firearms that may have fired a particular bullet or cartridge case.

Firearm Illustrations

Firearm Illustrations

Pump Shotgun

Auto Shotgun

Bolt Action Rifle

All cases that involve firearms identification start with preliminary examinations of the evidence for similar class characteristics.  Class characteristics can be defined as:

•Intentional or design characteristics that would be common to a particular group or family of items.

Such as:

•The class characteristics of firearms that relate to the bullets fired from them includes the caliber of the firearm and the rifling pattern contained in the barrel of the firearm.  •Cartridges and Cartridge cases on the other hand are examined for Unique level similarities in what are called breech marks, firing pin impressions, extractor marks, ejector marks and others.

Let’s look at some marks and impressions…..

•44 SPECIAL caliber cartridge cases fired

in a TAURUS revolver.

Breech Marks

•12 GAUGE shotshells

More Breech Marks

•38 SUPER AUTO cartridge cases fired in a COLT

pistol.

•357 MAGNUM caliber cartridge cases fired in a TAURUS revolver.

•357 MAGNUM caliber

cartridge cases fired in

a COLT revolver.

Firing Pin Impression22 LR caliber

rimfire cartridge cases fired in a RUGER pistol.

Action Marks

7.62x39mm cartridge

cases fired in an AK or SKS Type

rifle.

•Bullets collected for comparison to a specific firearm are examined first to see if they are of a caliber that could have been fired from the submitted firearm.  Then they are examined to determine if the pattern of rifling impressions found on the bullet match the pattern of rifling contained in the barrel of the questioned firearm. 

•If these class characteristics agree, the next step is to try to make a positive match between the unique characteristics that may have transferred to the bullet from the barrel.

•GRC Identification

•General rifling characteristics or GRC are the identified rifling pattern (i.e. 8/right) and the widths of the individual lands and grooves.in a barrel.

•What are Lands and Grooves?

The Pitch is the angle (tilt) at which the rifling is cut in the barrel.

Note these two bullets have different sized land impressions. They were fired from different weapons.

The Rate of Twist is referring to the distance the rifling will travel to complete a single complete turn.  Ex: 1 turn in 10 inches. 

CARTRIDGEMANUFACTURE TWIST L&G LAND GROOVE

9MM LUGER AA ARMS INC R 6 .055 .120

9MM LUGER ASTRA R 6

66666

.053 .128

9MM LUGER BERETTA R 6 .055 .130

9MM LUGER HI-POINT FIREARMS R 6 .055 .120

9MM LUGER INTERDYNAMIC R 6 .055 .124

9MM LUGER LLAMA R 6 .054 .120

9MM LUGER MAUSER R 6 .054 .128

9MM LUGER SMITH & WESSON R 6 .056 .122

9MM LUGER STAR R 6 .054 .126

9MM LUGER SWD INC R 6 .055 .120

LandImpression

GrooveImpression

•The images below show rifling impressions on a 32 caliber bullet at progressively increasing magnifications.

•A submitted firearm will be fired several times using a water tank to obtain standards from the firearm. 

•To make these comparisons the firearm examiner will use a comparison microscope.

EvidenceBullet Land ImpressionsMatch WithThose OnTest FiredStandardBullet.

What about a fragment?

Can you still determine the GRC of the weapon?

Maybe... if you have at least one Land and one Groove impression.

You would divide the 0.357 by the sum of the two impressions and then multiply that value by pi.Like this....

Let’s say you suspect the weapon to be a 0.357 handgun.

If the Land impression measures 0.055” wide and the Groove impression measures 0.130” wide...

0.055” Land

+0.130” Groove

0.185” Total

So...if Pi = 3.14 and the suspectedweapon is 0.357then the formula becomes...

0.055” Land

+0.130” Groove

0.185” Total

( 0.357 / 0.185 ) 3.14 = 6.05So total L & G should be ~6

Firearm examiners routinely examine a shooting victim's clothing for bullet holes and other evidence that may allow for a determination of the distance from the muzzle of the firearm to the clothing.

•There are tests to determine the firing distance by examining the area around a bullet entrance hole for gunshot residues

•These examinations are sometimes referred to as muzzle-to-garment or muzzle-to-target distance tests.

•Because the various elements included in gunshot residues are very small and lack mass they lose their energy rapidly.

•Gunshot residues can also be emitted from areas of a firearm other than the muzzle.

•The further gunshot residues travel from the muzzle, the broader and less concentrated the pattern becomes.

•Gunshot Residue can only travel between 3 and 5 feet out from the muzzle. 

Gun Shot Residue & Testing For Distance From Muzzle…..

Gunshot residue distance standards are made by firing the firearm, using ammunition like that used in the actual case, into witness panels that consist of  white pieces of cotton twill jean cloth.

How do we prove that the particles around this bullet entrance hole are GSR?

•The first chemical test conducted is called the Modified Griess Test

•The Modified Griess Test is a test to detect the presence of nitrite residues.

•The Modified Griess Test is the primary test used by firearms examiners to determine a muzzle-to-garment distance.

•The exhibit being processed is placed face down against a piece of treated photo paper, with the bullet hole centered on the paper.  

•The back of the exhibit being examined is then steam ironed with a dilute acetic acid solution in the iron instead of water. 

•The acetic acid vapors will penetrate the exhibit and a reaction takes place between any nitrite residues on the exhibit and the chemicals contained in the photographic paper.

The nitrite residues will appear as orange specks on the piece photographic paper.  

•A close-up of the results.

Q: What chemical test would be used to detect any lead residues present on the exhibit?

A: The Sodium Rhodizionate Test.

Step #1

Spray with SR + water solution

Q: What chemical test would be used to detect any lead residues present on the exhibit?

A: The Sodium Rhodizionate Test.

Step #2

Residue turns dark yellow/orange.

Q: What chemical test would be used to detect any lead residues present on the exhibit?

A: The Sodium Rhodizionate Test.

Step #3

Spray on a buffer solution

Q: What chemical test would be used to detect any lead residues present on the exhibit?

A: The Sodium Rhodizionate Test.

Step #4

The background color fades & pink appears

Q: What chemical test would be used to detect any lead residues present on the exhibit?

A: The Sodium Rhodizionate Test.

Step #5

Blue = Lead

Add dilute Hydrochloric Acid solution

•The above images show a car's headrest with a sooty deposit around a bullet entrance hole.  The right image shows the headrest after processing with Sodium Rhodizionate and a buffer solution.  

Typical contact entrance hole

Bullet hole (angled)

Angled grazing bullet holes made by a single bullet.

•Bullet Exit Holes

•Typical Bullet Exit Holes

Another Typical Bullet Exit Hole

The “Hand”gun of tomorrow ?????