Rifling Impressions

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.  As a result, a rifled barrel will impress a negative impression of itself on the sides of the bullet like those seen below.  

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.

The widths of the lands and grooves on a bullet provide a further class characteristic that can be used as a preliminary means to determine if the submitted bullet could have been fired from the submitted firearm.

Another class characteristic of rifling that is seldom comes into play is the rate of twist or pitch of the rifling in the barrel.  The rate of twist is the distance the rifling needs to spiral down the barrel for it to complete a single revolution.  An example would 1 turn in 12 inches.  The term pitch refers to the angle at which the rifling is cut in the barrel.  The two images below show the rifling in a 5 inch barrel on the left opposed to a 1 3/4 inch barrel on the right.  Note the difference in pitch of the rifling.  

The barrel on the right actually has very little pitch to the rifling.  Depending on which way you look at it the direction of twist could be to the right or left.  It is a 10/right rifled barrel.

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.  Unless there is a noticeable difference in the pitch, it can be hard to use this class characteristic as a means of elimination.  As a result, firearm examiners rarely measure the rifling impression pitch.  The two bullets below were fired from different 6/right rifled barrels but the pitch is different.  Not by much though and any damage to a bullet like this could make the difference difficult if not impossible to see.

Firearm examiners can run into problems determining any of the previously described class rifling characteristics on the bullet if the bullet is damaged like the one seen below.

You may get a bullet fragment that only has one or two land and groove impressions and the direction of twist may not be obvious.  

Therefore, here is where a little math comes in. Let us say you have a fragment with only one land and one groove impression visible (the minimum number for this to work).  You measure the land impression width to be .055 inches and the groove impression width to be .130 inches.  Divide the diameter of the bullets suspected or measured caliber, in this case .357, by the sum of the width of the one land and groove impression (.185), and then multiply that number by pi (3.14).  

.357 / .185 * 3.14 = 6.05

This will give you the approximate number of lands or grooves that would have been in the barrel that fired the bullet.  In this example, the approximate number of lands and grooves would be six.

If a bullet is badly damaged and exhibits poor class characteristics not all is lost.  There is still a possibility that some unique microscopic marks from the barrel still exist on the surface of the bullet.   

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