Bullet Basics 1- Caliber

When a bullet is submitted for comparison to a firearm, one of the first examinations conducted will be to determine the bullet's caliber.

Caliber is a term used to indicate the diameter of a bullet in hundredths of an inch.  A bullet that is 30 hundredths of an inch (.30) in diameter is called a 30 caliber bullet. The term caliber is of English origin and is used by ammunition and firearm manufacturers in the United States. Firearms and ammunition of European origin use the metric system and would refer to a 30 caliber bullet as an 7.62mm bullet. 

The caliber of the bullet is just the first class characteristic that must agree with the questioned firearm.  The bullet must also be of the type found in cartridges that the firearm will fire.  A cartridge is a single unit of ammunition consisting of the cartridge case, primer, and propellant with or without one or more projectiles.  The image below shows these various components.

Cartridges are usually given a name or cartridge designation by their developer, who is more often than not the manufacturer of a firearm (It doesn't make much sense to develop a cartridge if you don't have a firearm to fire it in).  The cartridge designation typically includes a numerical value to indicate the approximate diameter of the bullet and will often include the manufacturer's name.

It never fails that when a cartridge is developed the manufacturer or others will immediately try to improve on the design.  Variations to the original cartridge are usually inevitable and may be in the form of a longer or shorter case, differences in gunpowder, or differences with the weight and type of bullet contained in the cartridge.  For more information on bullet characteristics click here for Bullet Basics.

When a variation in the cartridge case length occurs the cartridge's name may include the term Short, Long, or Magnum to denote the difference.  Examples of which are the 32 S&W and the 32 S&W LONG cartridges seen below.  SMITH & WESSON developed both of these cartridges for use in their 32 caliber revolvers.  As you can see, the 32 S&W Long cartridge on the right has the longest cartridge case.  

Long or magnum cartridges may also have a heavier bullet when compared to the standard or short versions.  The 32 S&W cartridge above contains an 88 grain bullet and the LONG version contains a 98 grain bullet.  

Other variations can occur in the bullet weight and bullet construction within the same cartridge designation.  All of the cartridges seen on the right are in the 22 caliber "family" yet each has a different cartridge designation. From the left they are: 22 Blank, 22 Short, 22 Long, 22 Long Rifle Shot, 22 Viper, 22 Long Rifle, 22 Stinger, 22 Magnum, and 22 Maximum.

Cartridges designed for use in auto loading pistols will usually have the word AUTO in their cartridge designation (i.e. 32 AUTO, 45 AUTO).  Cartridges can also carry a +P designation that stands for plus power. Cartridges with the +P designation usually have no external differences from the lower powered varieties but contain different types of gunpowder to achieve higher velocities.

Measuring the bullet's diameter, weighing the bullet, and examining the physical characteristics of the bullet help firearm examiners to arrive at a basic caliber for the submitted bullet.  Firearm examiners also can compare the questioned bullet to known reference standards. Most firearms sections maintain an ammunition reference collection and manufacturers catalogs that can be used as reference material in determining a bullet's caliber.

Sounds simple doesn't it?  Well.... not so fast!

It's one thing to look at the bullet above (looking down on the bullet with a mirror in the back showing the bullet's profile) and to say "that's a Federal 9mm Hyda-Shok bullet," when more than likely you've got a bullet from the victim like the one on the right.  Is the bullet from a 9mm LUGER, a 38 SPECIAL, a 380 AUTO, or a 357 MAGNUM cartridge?  You get the idea.  Sometimes firearm examiners can be very specific but there are times when we just can't narrow things down to one particular caliber or cartridge.

To further confuse the issue, firearms are normally designed to fire a specific type of cartridge.  However, some firearms chambered to fire one cartridge can also fire another.  One of the most common examples is that a revolver chambered for 357 MAGNUM cartridges can also fire a 38 SPECIAL cartridge.  However, a firearm chambered for 38 SPECIAL cartridges cannot fire 357 MAGNUM cartridges.  This must be considered in examining the submitted bullet.  You don't want to automatically eliminate a bullet based on its caliber until you determine the varieties that may be fired in the questioned firearm.

If the caliber of the bullet submitted for examination matches the caliber of the submitted firearm or if we just aren't sure of the bullet's caliber, the firearm examiner will look for additional class characteristics in the form of rifling to further narrow their search.  Click the Next button below to continue.


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