Bullet Identification

Bullets 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.

Located within the rifling impressions on a bullet can be microscopic striations or scratches like those seen on the bullet below.   They sort of look like a bar code don't they?

Imperfections in the surface of the interior of the barrel leave striations on the projectiles.  Striations have the potential to be consistently reproduced in a unique pattern on every bullet that passes down the barrel of a firearm.  The key word in the previous sentence is unique.

Firearm examiners will attempt to find this unique pattern by following the procedures outlined below.

Examinations conducted

A submitted firearm will be fired several times using a water tank like the one on the left to obtain standards from the firearm.  Lids on the tank are closed and locked and the muzzle of the firearm is placed in the open tube at the end of the tank and fired.  Friction from passing through the water slows the bullets down and they end up on the bottom of the tank about halfway down its length.  The tank is approximately 3 feet wide, 10 feet long and 3 feet high.

Fired standards, like those to the right, are examined first to determine if in fact the barrel is producing striated marks in a unique and consistent pattern. Once a consistently reoccurring pattern to the marks is identified on standards, the standards are compared to the evidence bullets to see if the same pattern of marks exists on the evidence. To make these comparisons the firearm examiner will use a comparison macroscope (below right).

Notice that this is called a macroscope and not a microscope.  Microscopes typically use objectives that are 100x and above. Magnifications typically used in firearms identification are 5X, 10X, 20X, 30X, and 40X.  It is not unusual however to see these lower powered scopes referred to a microscopes.  In fact if you see it referred to as a microscope on this website just ignore it!

All firearm sections will have a comparison macroscope.  The comparison macroscope consists of two macroscopes mounted side by side and connected by an optical bridge. There are two stages on the lower part of the macroscope that the bullets to be compared are mounted on.  The bullets are  attached to the stages using some type of sticky substance.  Images of the bullets travel up through the objectives, bounce off several mirrors in the optical bridge, and are combined in a round field of view seen by looking into the stereoscopic eyepieces.  The resulting image will show the bullets mounted to the stages, side-by-side, with a thin dividing line down the middle.    The images below show rifling impressions on a 32 caliber bullet at progressively increasing magnifications.

The stages that the bullets are attached to allow the bullets being examined to be rotated on their axis and moved up, down, to the left, and to the right.  The bullets are rotated around to see if any microscopic similarities are present. Most positive identifications are made on striations that occur in land impressions and the best marks are usually near the base of the bullets like those seen below.

For an animated and interactive demonstration of this process see the 3-D Bullet Identification Demo.

Not all bullet identifications are like those seen in the above image.  Firearm examiners will examine the entire bullet for striations that agree with the standards.  Bullets can have as many as six, eight or even twenty-two different land and groove impressions and each one may have areas of agreement between the striations.  Taking an image of striations, like the one seen above will usually not be representative of the actual overall positive identification.  It really comes down to the experience of the firearm examiner and what they perceive to be the overall uniqueness of the striations that are present.

One of the biggest problems in making an identification is that few evidence bullets are submitted intact.  Most are badly distorted, wiped and/or fragmented.  The fragment seen below may not look like much but even small fragments and badly damaged bullets can still retain sufficient marks for an identification to be made.

Until the questioned bullet is examined microscopically by a trained firearm examiner you just don't know if it has marks of comparative value.  The comparison image below shows the above bullet fragment (right) compared to a standard (left) fired from the submitted firearm.

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