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.