Ballistic Testing

In 1784, someone using a flintlock pistol shot Edward Culshaw. In those days, there were no bullets, as we know them. Gunpowder and a ball of lead were put into the gun's muzzle and packed with paper wadding. A spark made when the gun's hammer struck some flint at the back end of the barrel ignited the powder. When the constable examined Culshaw's wound, he found a piece of newspaper used as wadding to pack the powder in the killer's gun. The prime suspect in the killing was a man named John Toms. When a piece of newspaper found in Toms' pocket was compared with the piece found in the wound, the pieces fit together like two pieces of a jigsaw puzzle. Based on the evidence, Toms was easily convicted. The Toms case was probably the first in America in which ballistics was used to solve a crime.

Much like in Tom's case, most investigations start with a crime having been committed. Forensic ballistics and firearm investigation start when there are bullets, cartridges, a weapon, or any combination of the above found at a crime scene. With the evidence, a crime lab can search for clues on these items that could lead to a suspect or possibly prove that the items were used in the crime. By comparing the markings on bullets or cartridges found at the scene with those fired from a suspect's weapon, a ballistics expert can often determine if the rounds came from the same weapon. Just the act of cycling a cartridge through a weapon without firing it can leave permanent scratches in the case that are unique to the weapon.

When a suspect's weapon is examined in the lab, it will be test fired into a box filled with cotton or a tank of water to provide the examiner with the bullets and cartridges with a known history. Using a microscope, the known cartridges are compared with the ones in question. With some patience, skill, and a little luck, experts can definitively say that a certain firearm and no other fired this bullet, or ejected this cartridge.

The value of luck cannot be overstated. A bullet may leave the muzzle of a weapon at over a thousand feet per second and slam into a concrete wall. This may deform the round beyond all recognition. Just about anything can and does happen to flying projectiles. Soft lead bullets are known to flatten out when they hit hard objects or even when they just hit plain water. Copper jacketed bullets may come loose from their jackets or disintegrate into numerous sharp pieces.

When a cartridge is fired, the pressure generated, which may be anything from two to over twenty tons per square inch, forces the case back against the breech face. This causes the individuality of the breech face to be imprinted on the base of the cartridge. In other words, the firearm leaves it's "finger-print" or "thumb-mark" is imprinted naturally varies with the pressure, but its image will always be present even though it is sometimes difficult to detect.

The bullet travels down the length of a gun barrel, and the hardened steel of the barrel wears the softer lead or copper of the bullet away ever so slightly. Tiny imperfections, which were introduced into the rifling, the gentle twisting groove pattern scribed into a gun barrel, when it was first manufactured, are created in the surface of the bullet. These markings are also unique as a fingerprint for the weapon. Guns of the same make produce striations, the scratch-like impressions left on the projectile, which are similar but not identical. There are usually several places where the hard steel of the weapon leaves its mark on the softer metal of the ammunition. The bullet is usually made of a lead or jacketed copper, both of which are softer than steel. The cartridge that remains in the gun will also have scratches and impressions made by the firing pin and extractor.

When a bullet or a casing is collected from a crime scene, examiners can often assist investigators by observing details about it without waiting for a firearm to be discovered. With a few exceptions, the direction of twists and number of lands and grooves impressed into the bullets or the nature of the scratches left in a cartridge case can give investigators an idea of which make of weapon might have fired it. A direction of twist refers to the right or left hand spin given to the bullet as it exits the barrel. Grooves are the cuts in the barrel form "lands", the term for the metal ridges remaining between the grooves. It is the lands that grip the bullet and give it a twist as it speeds through.

Smith & Wesson guns are known for having five lands twisting to the right. Colts are known to have six to the left. But unfortunately, there are exceptions to this rule. To determine the direction of twist, one would point the bullet away from them and look down from above it. The marks on a fired bullet having a right twist will angle to the right as they go from the base to the nose. Left twist bullets have marks angling to the left. Then the number of marks around the whole bullet is added up. If two rounds are placed base to base, examiners can sometimes tell at a glance if they should be looking for more than one firearm. If the lands do not match up in twist and number, or if the bullets are of different caliber, there is no point in trying to compare them microscopically, for this usually takes great effort.

In the late 1920's, Phillip O. Gravelle invented a microscope through which the images formed by two lenses could be viewed side by side through a single lens, and