What’s the Difference Between Rimfire and Centerfire Ammunition?

by Tommy Grant

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Are you a newbie when it comes to shooting? There are millions of new, first-time gun owners out there who have come to own a firearm in the last three-plus years. Are you confused by the terms ‘rimfire’ and ‘centerfire’ when it comes to ammunition? We were all in your shoes once. So don’t fret…here’s the skinny.

For the old salts who already know this stuff, please feel free to go directly to the comments. The author also wishes you to know that he will pretend that his feelings are hurt, if it makes you happy.

Anyhow, what are the differences between these two types of ammunition? There are a few, but first we have to explain how a cartridge works.

The simplest version is that an object — either a striker or a firing pin — strikes the back of a cartridge case (what some people mistakenly call a bullet). This agitates a tiny amount of a volatile self-oxidizing powder and causes it to explode. This is called the priming compound.

 

That small primer explosion produces a tiny spark which ignites a larger amount of powder (the main propellant charge or gunpowder). The propellant explodes, producing exhaust gases. The exhaust gas, having nowhere to go initially, propels the bullet out of the cartridge, down the barrel and toward the target/attacker/deer…whatever you’re shooting at.

 

 

 

How that primer is installed is the difference between rimfire ammunition and centerfire ammunition.

 

 

In rimfire rounds — most people are familiar with common .22LR cartridges — the rim of the case actually has a small space about as thin as a piece of paper. Think of it kind of like a filled pastry or a dumpling like a ravioli. In that space, a filling — the primer compound — gets smushed into the edges, which get crimped down when the cartridge is made.

 

 

What actually happens is a tiny dollop of wet primer compound is dropped into the bottom of case. The case is then spun until the primer compound collects around the rim, filling up the small space around the inside. The rest of the case then gets a bit of powder, a bullet is seated over it, then the whole thing gets crimped to hold it together.

To discharge it, a gun’s firing pin strikes the rim of the cartridge, making it go bang and setting off the round. Hence the term rimfire.

 

Centerfire cartridges work a little differently. The bottom of the cartridge case has a small hole punched in the center. Into that hole is pressed a primer cap with a small amount of primer compound in it.

After this primer cap is inserted, the rest of the cartridge is assembled. The propellant charge (gunpowder) is put in the case, the bullet is seated and the whole thing gets crimped together.

To fire a centerfire round, a striker or firing pin hits the primer cap in the center of the cartridge. That causes the primer compound to ignite, sending a spark into the main propellant charge. That explosion propels the bullet, going through the same process described above. Boom, gases, bullet, you missed the deer. Again.

Why are most cartridges centerfire, though?

It wasn’t always that way. In the 19th century, there were a number of rimfire cartridges that saw heavy use by civilians and the military alike. The Henry rifle was initially produced chambered for the .44 Henry, a rimfire rifle round, and the Spencer rifle was chambered for .56-56 Spencer.

Other rimfire rounds were popular too, such as .44 Rimfire – a pistol round – and a few others, but only a few have hung on in the fullness of time.

Centerfire rounds, however, quickly gained the upper hand and for very good reasons.

First, centerfire ammo is generally more reliable. Rimfire rounds — for various reasons — seem to be more prone to duds.

The manufacturing process for centerfire rounds is much easier and (it must be said) safer to boot, since the first part of making a rimfire round is agitating the case with a volatile primer inside. Centerfire cases can also withstand much higher chamber pressures, which means cartridges can be made with MOAH POWAHHH! and that’s a valuable thing.

Also, rimfire ammo is not reloadable because the casing rim gets completely damaged during the firing process. Centerfire ammo, on the other hand, is reloadable because the primer is set above the base of the cartridge,  and centerfire ammunition casings are not rendered useless after firing.

Centerfire primers also come in two basic types: the Berdan and the Boxer. The Boxer primer has a single narrow channel through which the spark ignites propellant. In the Berdan primer, the spark from the primer enters the cartridge through an anvil-shaped block in the middle of the primer with two very narrow channels on either side (See illustration above).

However, there is a limit to centerfire rounds, specifically in size. Once you get down to about .23 caliber in a non-rifle round, that’s about the limit of how small a human or mechanical hand can easily and repeatedly make a cartridge. Today, the smallest you can get with any frequency is .25 ACP and even then not terribly often. In previous eras, the smallest was the 5.5mm Velo Dog, a small caliber literally invented (not making this up) for a pocket handgun designed for French bicyclists so they could shoot aggressive dogs.

Centerfire ammo does better in larger calibers however. And that makes it good for self-defense, military and long-range applications, as well as big-game hunting

However, a few rimfire rounds that were mighty popular about the time that centerfire cartridges took off — smaller calibers that were popular for shooting squirrels and such — hung around due to their usefulness for small-game hunting and as target rounds. This was the .22 rimfire ammo family, including .22 Short, .22 Long, and .22 Long Rifle with the lattermost being one of (if not) the most popular cartridges in production. The .22 Winchester Magnum Rimfire came along in 1959, too, and now there’s also .17 caliber rimfire rounds for shooting varmints and plinking.

So, those are the basics of rimfire vs centerfire. The former is, today, limited to smaller rounds for marksmanship, varminting and hunting small game. The latter does that and more.

 

 

More from The Truth About Guns

How to Get Started Reloading Rifle Ammo
Why Range Ammo Is A Terrible Choice For Self-Defense

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