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Firearms do not normally change much over time. This allows for firearms recovered months or even years after a shooting to be identified as having fired a specific bullet or cartridge case. Tests have been conducted that found that even after firing several hundred rounds through a firearm the last bullet fired could still be identified to the first. It should be noted that not all firearms leave consistent reproducible marks. But overall it has been my experience that around eighty percent of the firearms that I examine produce what is sometimes called a "mechanical fingerprint" on the bullets and cartridge cases that pass through them. All cases that involve firearms identification start with preliminary examinations of the evidence for similar **//class characteristics//**. Class characteristics can be defined as:
 * //S//**tudies have shown that no two firearms, even those of the same make and model, will produce the same unique marks on fired bullets and cartridge cases. Manufacturing processes, use, and abuse leave surface characteristics within the firearm that cannot be exactly reproduced in other firearms.

//**Intentional or design characteristics that would be common to a particular group or family of items.**//

A very basic example would be that several no. 2 pencils in a box are yellow and have pink erasers. The color and eraser type is a common class characteristic to all of the pencils. When it comes to firearms and ammunition it is not quite so simple. The class characteristics of firearms that relate to the bullets fired from them includes the //**caliber**// of the firearm and the **//rifling//** pattern contained in the barrel of the firearm. Cartridges and Cartridge cases on the other hand are examined for class similarities in what are called **//breech marks//**, **//firing pin impressions//**, //**extractor marks**//, **//ejector marks//** and others. If dissimilarities in class characteristics are found or if a general lack of good class characteristics are present no further comparisons may be necessary. When similar class characteristics are identified the examinations progress to a final stage where an attempt is made to find a "match" in what are called **//individual characteristics//**. //**The AFTE Glossary defines I****ndividual characteristics as:**//

//**m****arks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or caused by use, corrosion, or damage. They are unique to that tool and distinguish it from all other tools.**//

The transfer of individual characteristics from a firearm to the ammunition components passing through it is what makes firearms identification possible. Click the **//Next//** button below learn more about **//[|bullet identification]//**. 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?
 * //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.

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 **//macro//**scope and not a **//micro//**scope. 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.
 * [[image:http://www.firearmsid.com/jpgs/990900192_small.jpg width="103" height="76" align="center"]] ||
 * [[image:http://www.firearmsid.com/jpgs/990900193_small.jpg width="101" height="76" align="center"]] ||
 * [[image:http://www.firearmsid.com/jpgs/990900194_small.jpg width="100" height="76" align="center"]] ||

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.

Results When comparisons are made between firearms and fired ammunition the results can read as follows:

This conclusion is reached after all class characteristics agree and a sufficient correlation between individual characteristics is found. This conclusion is reached if class characteristics agree but there is an insufficient correlation between individual characteristics. This conclusion is reached if class characteristics disagree.
 * //Exhibit 1 (bullet) was identified as having been fired from Exhibit 2 (revolver).//**
 * //Exhibit 1 (bullet) could neither be identified nor eliminated as having been fired from Exhibit 2 (revolver). All comparisons were inconclusive.//**
 * //Exhibit 1 (bullet) was not fired from Exhibit 2 (revolver).//**

Additional examples of bullet comparisons can be found in the [|**//image galleries//**]. In some cases, a firearm may not be recovered for comparison. When this happens firearm examiners can examine bullets for **//[|general rifling characteristics (GRC)]//** in an attempt to determine what brands of firearms from which the bullet may have been fired. Check the GRC links above for more information on this type of examination. We have now discussed how bullets can be identified as having been fired from a firearm but what about the cartridge cases. Click the **//Next//** button below to learn about **//[|Cartridge Case Identification]//**. Cartridge cases like those on the right are mostly made of brass but can also be made of other materials such as steel and plastic. Cartridge cases come in a variety of finishes but all are made of a material that is softer than the materials found in a firearm. Any surface of the cartridge case that meets the inner workings of the firearm may be marked. Tool marks produced on the cartridge cases will be in two basic forms. As the microscopic striations found on bullets, cartridge cases can pick up //**[|striated action marks]**//. These "scratches" are produced when the cartridge case moves laterally against the tool (inner surface of the firearm) producing a scrape or //striated// mark. The other form of marks that can be left on a cartridge case are **//[|impressed action marks]//**. Impressed marks are created on cartridge cases when it impacts the tool (again, the firearm) with adequate velocity or pressure to leave an impressed or indented mark. Cartridge cases are compared to fired standards from a firearm using a comparison microscope as described on the //**[|bullet identification]**// page. Standards are first examined to determine what marks, if any, the firearm is consistently reproducing. Evidence cartridge cases are then directly compared to the standards to see if they too are also similarly marked. Cartridge case comparison results may be reported as follows:
 * //L//**ike bullets, cartridge cases can be identified as having been fired by a specific firearm. As soon as cartridges are loaded into a firearm the potential for the transfer of unique tool marks exists. However, the cartridge does not have to be fired for these marks to be transferred. Simply //loading// a cartridge into a firearm can cause unique identifiable marks that can be later identified.

The above conclusion is reached if the action marks present on the questioned cartridge case are determined to be because of the actual firing process. An example of which can be breech marks as seen in the comparison image below.
 * //Exhibit 1 (cartridge case) was identified as having been fired by Exhibit 2 (firearm).//**



The above conclusion is reached if the cartridge case lacks sufficient action marks to be identified as having been fired by the questioned firearm or the firearm in question fails to produce reproducible individual characteristics on standards. All general class characteristics such as caliber and firing pin shape would have to agree. The image below shows a comparison between two cartridge cases that lack any individual characteristics but have a similar general appearance.
 * //Exhibit 1 (cartridge case) could neither be identified nor eliminated as having been fired by Exhibit 2 (firearm).//**



This conclusion can sometimes be reached when the submitted cartridge case exhibits very good individual characteristics that are very dissimilar to those produced on standards. However, consideration must be given to the possibility that the firearm in question could have changed significantly. If all dissimilarities can be accounted for, a negative conclusion will be reached. The comparison image below shows two cartridge cases that exhibit noticeably different breech marks and firing pin impressions.
 * //Exhibit 1 (cartridge case) was not fired by Exhibit 2 (firearm).//**



This conclusion can be reached if the cartridge case is found to have action marks that result from simply loading and/or unloading a cartridge case in a firearm. The comparison image below shows striated action marks on the shoulder of cartridges that have been loaded and unloaded in a Chinese AK Type assault rifle.
 * //Exhibit 1 (cartridge case) was identified as having passed through the action of Exhibit 2 (firearm).//**


 * //S//**triated action marks are common to cartridge cases that have passed through the action of an auto loading or repeating firearm. Striated action marks can be produced on cartridge cases by contact with a number of different areas within the firearm. Some of the more common striated action marks include chamber marks; shear marks, firing pin drag marks, extractor marks, and ejector marks.

Chamber Marks One of the most common striated action marks are called **//chamber marks//**. Roughness in the chamber of a firearm can scratch the outer walls of a cartridge case when loaded and removed from the chamber. Most chamber marks occur after the cartridge is fired. Cartridge cases expand when fired pressing out against the walls of the chamber. When they are pulled out of the chamber, the sides of the cartridge case can be scratched. The comparison image below shows chamber marks on 22 caliber, rimfire cartridge cases.

Shear Marks Another common striated action mark are **//shear marks//** produced by GLOCK pistols on cartridge case primers. GLOCK pistols have a rectangular firing pin hole (below) in their breech face.

When a cartridge case is forced backwards from recoil the primer imbeds itself in the firing pin hole. As the slide of the pistol starts to recoil, the barrel will drop slightly as the action opens. The dropping barrel forces the cartridge case to move down slightly and when this happens the lower edge of the imbedded primer is sheared downward and out of the firing pin hole. The resulting striated marks can be seen in the comparison image below.

Firing Pin Drag Marks In a similar process, striated marks called **//firing pin drag marks//** can be produced. When the firing pin springs forward to strike the primer of a cartridge, it may remain slightly forward and imbedded in the primer. Certain barrels (like in the GLOCK) drop down slightly as recoil is forcing the action open. The cartridge case drops with the barrel causing the nose of the protruding firing pin to drag across the primer as it leaves the firing pin impression. The below comparison image shows firing pin drag marks produced by a Colt 45 AUTO pistol.

Extractor Marks Another action mark, usually found in a striated form, are those created by the **//extractor//** of most auto-loading or repeating firearms. The extractor is a small part sometimes resembling a hook that is used to remove a cartridge or cartridge case from the chamber of a firearm. The image below shows the extractor of a 9mm GLOCK pistol hooked into the extractor groove of a cartridge. As the slide of the pistol moves to the rear, the extractor pulls the cartridge case along with it until it is ejected from the pistol.

The extractor may or may not leave an identifiable mark on the cartridge case. This is true if the cartridge is fired or simply hand chambered and extracted without firing. Extractor marks may look like those seen in the comparison image below.

Ejector Marks As described above, the extractor pulls the cartridge case out of the firearm's chamber. As the cartridge case is pulled to the rear it will be struck somewhere on an opposing edge by a part as seen below called the **//ejector//**.

The ejector is designed to expel the cartridge case from the action of the firearm. The resulting impact of the cartridge case with the ejector will cause another action mark that can be used as a means of identification.

Ejector marks can be striated in nature but a lot of the time they are **//impressed action marks//**. Click the next button below to learn more about impressed action marks. Additional examples of cartridge case comparisons can be seen in the [|image galleries]. //**I**//mpressed action marks, with a few exceptions, are produced when a cartridge case is fired in a firearm. The two most common impressed action marks are **//firing pin impressions//** and **//breech marks//**. As mentioned at the end of the Striated Action Marks page, //**ejector marks**// can also be in the form of an impressed action mark. Firing Pin Impressions The comparison image below shows the firing pin impressions on two centerfire cartridge cases. As you can see, the firing pin impressions have both circular manufacturing marks and parallel marks from a defect in the nose of the firing pin.
 * //Firing pin impressions//** are indentations created when the firing pin of a firearm strikes the primer of centerfire cartridge case or the rim of a rimfire cartridge case. If the nose of the firing pin has manufacturing imperfections or damage, these potentially unique characteristics can be impressed into the metal of the primer or rim of the cartridge case.

The comparison image below shows firing pin impressions on two rimfire cartridge cases. Imperfections in the surface of the nose of the firing pin consistently produced these impressed marks.

Firing pin impressions also can be found on live cartridges. One of those few exceptions I mentioned earlier. In some cases, the firing pin may miss the primer of a cartridge or fail to strike the primer of a cartridge with sufficient force for it to discharge. The cartridge may also misfire due to a contaminated or deteriorated primer compound. For whatever reason, the result will be the presence of a firing pin impression on the cartridge case of a live cartridge. This could be significant if the cartridge is say, left at the scene or found at a suspect's house. The comparison image below shows light firing pin impressions on an evidence cartridge case (left) and a test standard from a suspected firearm (right).

Breech Marks By far the most common impressed action marks on cartridge cases are **//breech marks//**. Most fired cartridge cases are identified as having been fired by a specific firearm through the identification of breech marks. Very high pressures are generated within a firearm when a cartridge is discharged. These pressures force the bullet from the cartridge case and down the barrel at very high velocities. When a firearm is discharged, the shooter will feel the firearm jump rearward. This rearward movement of the firearm is called **//recoil//**. Recoil is for the most part caused by the cartridge case moving rearward as an opposite reaction to the pressures generated to force the bullet down the barrel. When the head or base of the cartridge case moves rearward, it strikes what is called the **//breech face//** of the firearm. The image below shows the breech face of a 12 GAUGE, single-shot shotgun.

The breech face rests against the head of the cartridge case and holds the cartridge case in the chamber of the firearm. When the head of a cartridge case slams against the breech face, the negative impression of any imperfections in the breech face are stamped into either the primer of the cartridge case or the cartridge case itself. The image below shows the primer of a shotshell fired in the above shotgun.

Breech marks come in various forms. Those seen above are called parallel breech marks. Obviously, because the marks are a series of parallel lines. Another form of breech marks are circular breech marks like those seen in the comparison image below.

Breech marks can also show no obvious pattern. They may have a stippled or mottled appearance as seen below.

Ejector Marks Now back to **//ejector marks//**. Ejector marks are sometimes created when cartridges or cartridge cases are ejected from the action of a firearm. Ejector marks can be either striated or impressed but the impressed ejector marks not only can be used to identify a cartridge case as having passed through a firearm's action they can also be an indication that the cartridge case was fired in the firearm. Ejector marks like those seen below could only be reproduced when the cartridge cases were fired in the firearm and not by simply hand chambering and ejecting a live cartridge.

Additional examples of cartridge case comparisons can be seen in the [|image galleries].(TOREY C.)