Rifling, Striations and the Identification of a Firearm

In one line

A barrel writes on every bullet that passes through it, and the writing is peculiar to that barrel.

In the wording a student can write in an exam: the inner surface of a rifled barrel carries spiral lands and grooves which engrave the bullet as it passes; the number, width, direction and pitch of those marks are class characteristics shared by every barrel of that make and model, while the fine striations produced by the tool marks of manufacture and by subsequent wear, corrosion and damage are individual characteristics peculiar to the one barrel; and a firearm identification is made by test firing the suspect weapon and comparing the striations on the test bullet with those on the crime bullet under a comparison microscope, an agreement in a sufficient run of consecutive matching striations supporting the opinion that both were fired from the same barrel.

Class characteristics first

Number of lands and grooves. Four, five, six, eight, depending on the maker.

Direction of twist. Right or left.

Width of the lands and grooves.

Rate of twist, expressed as the distance in which the rifling makes one complete turn.

Calibre.

These are set by the manufacturer, so they are shared by every barrel of that make and model. What they establish is exclusion and class. If the crime bullet has six grooves with a right hand twist and the seized weapon has four with a left hand twist, the weapon did not fire it, and that conclusion is as strong as any in forensic science. If they agree, the weapon could have fired it, along with every other weapon of that model.

Individual characteristics

Where they come from. The tools that cut or formed the rifling leave microscopic irregularities, and no two barrels come off the machine identically. To that is added everything that happens afterwards: erosion from firing, corrosion from moisture and residue, and damage from cleaning rods and from accident.

What they look like. Fine parallel scratches, called striations, on the surface of the bullet, running along its length within the land and groove impressions.

Why they identify. The pattern of striations is a record of the surface of that barrel at the time of firing. The claim is that the pattern is unlikely in the extreme to be reproduced by a different barrel.

The comparison

Test firing. The seized weapon is fired into a medium that stops the bullet without marking it: a water tank or a cotton box. Two or three test bullets are taken.

The comparison microscope. Two microscopes joined by an optical bridge, so that the examiner sees two objects side by side in one field. The crime bullet and the test bullet are mounted and rotated until the striations line up.