fifthgroupthree


 * B**ullets 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. They are then 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 individual characteristics that may have transferred to the bullet from the barrel.


 * A** bullet is slightly larger in diameter than the **bore diameter** of the barrel in which it is designed to be fired. The bore diameter is the distance from one land to the opposite land in a barrel.

The rifling pattern in the barrel that fired a particular bullet can be determined by counting the number of groove or land impressions around the circumference of the bullet. Then, by holding the nose of the bullet pointing away from you, the direction the impressions run away from you (either to your left or right) determines the direction of twist. If the rifling impression pattern on the bullet matches the rifling pattern in the barrel of the questioned firearm, the next step is to measure the rifling impressions on the bullet. The lands and grooves on a bullet are measured in thousandths of an inch or in millimeters. One way to measure individual rifling impressions is to use a micrometer like the one below. The right image below shows the micrometer positioned next to a land impression on a bullet.

This is important because even though the rifling pattern may match between the bullet and questioned barrel one 6/right rifled barrel can have lands and grooves of a differing width than another. The image below shows the land impressions on two bullets. Both were fired from 6/right rifled barrels. The land impressions are lined up at the bottom edge but as you can see, the upper edges do not line up because the land impression on the right bullet is wider.

When bullets are compared to standards from a given barrel the pitch to the rifling impressions can be a means to eliminate the bullet as having been fired from the firearm. If the angle disagrees with the angle found on standards then the comparison will be a negative one based on those class characteristics. The problem with this is that it is hard to accurately measure the pitch. leann's slides...