Rimfire v. Centerfire

Terminology Lesson 2: Rimfire versus Centerfire

These terms refer to the way that the cartridge of the round.

In centerfire, the firing pin hits the primer, contained in a firing cap, in the center of the base of the catridge.  If you look at the casing after firing centerfire, you see a round divot in the center, as below.  Centerfire ammunition is the most common style for military and law enforcement.  Centerfire cases are made of thicker metal than rimfire, which allows them to withstand higher pressures, and thus can be fired at higher velocities.  In addition, most shotgun shells are centerfire, though they typically use a  plastic casing.  Centerfired casings can be reloaded (another topic for later, replacing the primer, gunpowder, and projectile.

From a 9mm Beretta 92FS

In rimfire, the firing pin strikes the rim of the cartridge base.  The rim acts as a percussion cap for the cartridge.  The firing pin leaves a notch type depression on the edge of the casing, as below.  Rimfire casings must be made out of very thin material to allow for the necessary deformation by the firing pin to ignite the primer.  This deformation means that the cartridge cannot be reloaded.  However, this thinner casing allows for rimfire ammunition to be much less expensive than centerfire.  Almost all modern rimfire guns are .22 caliber or smaller, though historically rimfire was used in firearms up to .44 caliber. 

From a .22 match rifle

Extra bit:
Semi-automatic vs. revolver ammunition.  Semi-automatic casings have an inset rim (but are generally referred to as rimless) that allows the extractor to remove the casing after firing.  Revolver rounds have a rim at the base to prevent the round from moving too far into the cylinder, which would prevent the firing pin from reaching the base and firing the round.  I'm on the hunt for a good picture of the difference, we don't have any revolvers in the house...



As always, I hope you learned something.  I know I did in writing this! What do you shoot?  Everything in our house is centerfire, but I got to play with a friend's .22 match rifles and scoped target pistol this weekend.  Tons of fun!

Do you have a terminology (or other) questions?  I'd love to write about anything y'all think is important.