Help!

Question
I know almost nothing about guns or rifles, but i wanted to know what do the different numbers an letters mean? i know mm is millimeter but what is it measuring? what about a 40 caliber? how do they come up with the number, and what does it mean? i know this is kind of stupid, but i would appreciate your expertise.

Answer
Sarah,

Yours is not a stupid question at all.  If you aren't familiar with firearms, concepts like caliber can be a bit of a mystery.  

Caliber is an approximate measure of barrel diameter in a firearm.  By logical extension, it is also an approximate measure of the diameter of the bullet traveling through a particular barrel.   

In most European countries, the bullet diameter is expressed in millimeters which is abbreviated as 'mm'.  Popular European pistol calibers include 9mm, 10mm and 7.65mm.

In the United States, we use primarily inches, not millimeters, so caliber is expressed as inches.  .40 caliber means that the barrel diameter (and bullet diameter) is approximately 40/100 of an inch.  .45 caliber would be 45/100 inches, .38 caliber would be 38/100 inches and so on.  

Many times you will also see cartridges designated with addition information that describes the 'chambering' information for a particular cartridge.  For example, .45 ACP refers to a .45 caliber bullet that is chambered as an Automatic Colt Pistol cartridge.  While this originally referred to a specific pistol, there are many guns made by multiple manufacturers that accept this type of cartridge.  A pistol cartridge designated as a .40 Smith&Wesson, or .40 S&W is again a specific caliber/chambering combination.  On the European side, the popular 9mm Parabellum or 9mm Luger is another way of saying that the caliber is 9mm and the case length is 19mm.  You will sometimes see the 9mm referred to as a "9x19" cartridge.

The designation that is used to describe different cartridge configurations is often a function of which manufacturer first introduced the cartridge.  ACP refers to 'automatic Colt pistol' which was introduced by Colt Firearms.  .308 Winchester was first introduced by Winchester Repeating Arms.  

Some military ammunition simple carries a designation that is more standard.  For example, the military designation for the .308 Winchester cartridge is:  7.62 x 51 NATO (numbers are referring to millimeters).

While there is some overall consistency, ammo references are not as consistent as one might wish to see.  This is largely due to the fact that different manufacturers have held different opinions about what caliber might work best in certain situations.  If you look at both rifle and pistol calibers, there are literally hundreds of different calibers and chambering out there.  It would be nearly impossible to know them all, but if you understand how they are described, many times you can figure things out...

Hope this helps you in your understanding of our sometimes arcane terminology.

Best regards,

Doug