Caught by a Bullet

Most of us are familiar with the term striations, and their association with ballistics analysis.  But how much to you really know?  Do you know exactly what’s involved with matching a bullet to a weapon, or where investigators start?

First of all Ballistics, or “Ballistics Fingerprinting refers to a set of forensic techniques that rely on marks that firearms leave on bullets to match a bullet to the gun it was fired with” (as quoted from Wikipedia).

The first thing that is determined is the caliber and type of bullet. It’s weighed, and even if it’s deformed, this can eliminate some calibers. They will also look at the type of jacketing to help identify the type of weapon used.  Rifles typically fire a fully jacketed bullet, whereas a small-caliber handgun fires a simple lead bullet.

Spiral groves are machined into the barrel of a gun to cause the bullet to spin and ensure an accurate projection.  As these tools are used and worn, the impressions made in the barrels of the guns vary.  The flat surfaces between these grooves are called "lands".  It's because of these factory-made impressions that investigators can determine which type of gun the bullet was fired from (this is considered class evidence).

As taken from “Howdunit Forensics A Guide for Writers”:

Colt type:  six broad grooves, narrow lands, and a left-hand twist

Browning type:  six broad grooves, narrow lands, and a right-hand twist

Smith & Wesson type:  five lands and grooves of equal width, and a right-hand twist

Webley type: seven broad grooves, narrow lands, and a right-hand twist

Marlin type:  Marlin rifles use a technique known as microgrooving, which leaves between eight and twenty-four narrow grooves inside the barrel.

While an investigator can tell the type of gun, it's not these impressions that individualize the fired bullet to the gun.  As the gun is used, the grooves and lands are affected.  They wear and get damaged as more bullets are fired through the barrel.  Soot and grit are left behind after the firing of a bullet, and this also alters the imprint left on the next bullet.

When the suspect weapon (class evidence) is determined, bullets are fired into a test-firing chamber.  This is then compared with the crime scene bullet.  Due to factors noted in the above paragraph, no two bullets (even fired by the same gun) are identical.  For there to be a match, there has to be identical patterns found on at least three consecutive striations on each bullet.

Interesting point:  If a silencer was used, and unavailable to the investigator, the bullet striations fired from the suspect’s gun (even if the guilty party's) will not match up.

Shotguns don't leave striations because they fire a wad of pellets (shot) rather than a single bullet.  The size of the shot can give information as to the ammunition used.

Shell Casings

Shell casings left at the scene can tell investigators a lot.  They can lead investigators to conclude the type of gun used to fire the gun.  If they have a suspect’s weapon for comparison they may even be able to match this to the gun.  When the firing pin strikes the bullet, the pressure drives the case against the breech block (which is the back wall of the firing chamber).  Because of the fact it’s hardened steel, the softer metal of the cartridge becomes imprinted with any imperfection.  Also, the firing pin leaves the shape of its own print on the cap, as does the ejector mechanism.

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Some resources: 

Howdunit Forensics A Guide for Writers

Ballistic Fingerprinting