jimtrue.com : school : CJT2141 : 15: Firearms, Tool Marks and Other Impressions
Posted by Jim True on April 23, 2003 6:37 AM. Last Updated October 22, 2006 9:23 PM
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Firearms -
Handguns, Longarms
Ammunition Components
-Casings
-Bullets (projectiles)
-Powders
Ballistics is the 'physics of the trajectory of projectiles'
Firearms Identification refers to "the discipline of forensic science which has as its primary concern to determine if a bullet, cartridge case or other ammunition component was fired by a particular firearm."
Ultimate goal is to get individual evidence in firearms identification; when dealing with cheaper made guns, the ammunition changes the barrell with every shot. Do not have that problem with American made weapons, but cheaper made metals in barrells would have that problem.
Recover the weapon and any ammunition with it. Want to use the same ammunition for test firings.
Determine the type of weapon: handgun, rifle, etc. By looking at the markings on the projectile and the cartridge casings, can determine caliber, by looking at markings can determine ammo manufacturer.
Weapons functionality: does the safety work? etc. Can this weapon fire the bullet how recently was it broken?
Gun Shot Residue: all a GSR does is prove that you were in the environment of a fired weapon.
Muzzle to Distance determination: ie made a 1 to 1 photograph of the stippling. Then used the weapon and same type of ammunition, put up white cardboard at 6", 2 ft, 5 ft, and fired the weapon and comparing the pattern to the 1:1 photograph.
Serial Number Restoration: strong acids on metal to determine if a serial number has been filed down. Can be anything.
Toolmark comparisons: prybars, screwdrivers, bolt cutters.
Always go first for the class and then see if you can get the individual characteristics.
Basic components of any firearm:
Barrell: rifled or smooth
Firing Chamber (single or multiple)
Stock (handle)
Action Mechanism for firing, loading and unloading
Basically this is a controlled explosion of two low explosives (one in the primer, then the gunpowder in the projectile).
Explosion forces the slide to go backwards and hit the spent casing against the extractor pin and cause it to ejected.
Gun Control: if guns were outlawed, only outlaws would have guns.
Single Action: pull the trigger, brings the hammer back and forward.
Double Action: requires the hammer to be pulled back and the trigger to be pulled to fire.
Backfire: barrell is plugged, explosion has to happen somewhere. Usually the gases are what is exploding back through the back of the gun.
Revolver: cylinder is rotating to the left or to the right everytime the trigger is pulled; that puts another cartridge in position so that when the hammer falls the projectile is fired. Spent casings are rotated out of the way.
Semi-automatic weapons used to jam a lot more often in the early days; mechanisms weren't built very well. Early 80's with the Drug Lords and more powerful weapons and the officers needed more fire power. 9-10 rounds in a .9 mm semiautomatic, as opposed to a 6 round revolver. Cartridge clips are faster in a semi-automatic weapon.
Rifling: pg 422; etched like a barber pole, grabs the projectile as it is coming down the barrell and forces it to spin. Gives it a truer flight path. The Lands and grooves are also creating and forcing a metal to metal impression on the projectile.
Figure 15-2 (pg 422), raised levels known as lands, lower parts are grooves.
On the Bullets: the Lands are the Low Marks, grooves are how
In the Barrell: The lands are the high marks, grooves are low
A lot of information can be found from these lands and grooves.
1st: look at the base of the Bullet
Twist - look for right or left hand twist.
[Right hand twist will eliminate Colt, they have left hand twist]
Count the Lands & Grooves: Some manufacturers will use 6 lands/grooves, 8 or 12, etc.
Measure the Widths of Lands/Grooves: narrow down the possibilities even more
Create a list of possible weapons to go look for
Not all firearms have Rifled barrels: Shotguns, Derringers and Musket shooters. DRT (Dead Right There). Smooth barrels.
Derringers, little hand held weapons 1,2 or 4 shots. 5 ft.
Rifling Methods: everytime you ream out the barrel with the rifling method it will change the cutter. By forcing the rifling onto the barrel, it changes the tool, which will make individual changes to the barrels.
Glock, not a plastic gun, but has a metal barrel.
Some bullets get mangled so bad that we don't have the entire area to attempt to identify.
Takes a year and a half apprenticeship program before the Firearms examiner can testify in court.
Give what is possible to determine. Bullet may be so mangled can only get a 'weight' of the projectile and determine a caliber.
Rifles bore diameters by Caliber, shotguns bore diameter measured by gauge. Except for .410)
Rifles single projectile; shotgun shell multiple projectiles.
Shotgun Shell: pellets, plastic or paper, wadding of paper separating gunpowder. Primer ignites gunpowder, forces the wadding out that forces all the pellets. Most effective at close range.
Recommend going to the firing range.
Action is different than Semi-automatic, etc.
Live round of ammunition is known as the 'cartridge'
Bullet: projectile fired from the weapon
Cartridge case (shell casing): container for gunpowder and primer, crimped at mouth to hold bullet.
Gunpowder: fast burning nitrocellulose propellant for projectile
Primer: shock sensitive charge which ignites gunpowder.
Bullets - Basic roundnose, wadcutter, hollowpoint
(Jacket means metal around the bullet, full metal jacket)
Hollowpoint, explodes outward (mushroom effect) very lethal because it hits and fragments.
"Black Talon" - pretty much go through anything; hollow point, jacketed with teflon material. Teflon material breaks out on impact with jagged edges like a circular saw, all those sharp edges will cut into it. It will go further into a bullet proof vest, but won't go into them. 'Little buzzsaws'
Cartridge cases, headstamp and firing pin impression. Factory manufacturer's indicate with caliber and manufacturer known as the 'headstamp'. also extraction marks on the side of the cartridge case.
Gunpowder under the microscope can have an appearance like a clover, beer barrell, or a diamond. Different shapes can effect the burning rate. [Unused gunpowder, not fired].
Smaller number means larger pellet with 'gauge'. Wadding usually helps to identify the ammunition manufacturer.
p.428 Shotgun shell components.
Anytime you can switch the barrell will be harder to identify the weapon; reaming the barrell can also make identification harder.
Stereo & Comparison Microscope - Basically a Comparison Stereo Microscope; 10-20x, 40x max. Don't have the two offset lenses, but dealing with stereomicroscopic magnifications.
Caliber diameter of the barrel.
Collecting the weapon, want to leave it in as close as possible condition it was in right after it shot.
Dropped in the river; leave it in a bucket (fresh water), let the lab dry out the weapon so it will not rust; salt water and oxidizing, probably ruined.
Don't clean the weapons, no pencils in the barrel.
Cartridge cases are harder to make id's on; bullet is the best identification.
EVERY WEAPON THAT COMES IN: Firearms Function Testing -
Is weapon capable of discharging ammunition?
Do safety mechanisms function as designed?
Will weapon fire accidentally?
Has the weapon been altered? (shortened, made to fire full auto?)
Sometimes check light or heavy trigger pull, how much weight
Test Fire in water will not leave marks; can use cotton, but most be damp or it will catch fire. Cotton can also leave striations.
Water tank about the size of our tables, 3/4 inch steel all around. Collect the spent projectile with pole with silly putty.
Energy every action has an equal and opposite reaction.
SEM is the method of choice in conjunction with EDX.
.22 rimfire doesn't contain barium or antimony.
Document serial number restoration immediately because the acids are incredibly powerful and will only show for a small moment. Photograph the end result, then the surface should be cleaned again to stablilize the serial number (lubricant). GET THE PHOTOGRAPHS.
Toolmarks - result from contact between a tool and some other surface.
Two types: Impressions or Striations (Impressions like on the bullets), Striations like on the Cartridge casings.
Also can find trace evidence.
Metal, castings, 1:1 photographs. Preserve the surface where the mark was made and take it back to the laboratory.
Striated Marks: Tool moved under pressure across surface, imperfections create striations. Piece of lead to reproduce striation marks.
Comparisons best carried out on the object itself.
DRUGFIRE (BulletProof); much like AFIS and CODIS, record the markings in
NIBIN - National Integrated Ballistics Identification Network
IBIS - Integrated Ballistics Identification System
DrugFire - FBI, casings
IBIS - ATF, Casings and Bullets
Treasury Dept. spearheaded NIBIN to combine the two systems.
No questions on Footwear (except maybe impressions); handled mostly under fingerprints.
Polygraph: Methodology, Responses
Questions at the end of the chapter.
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