Rimfire and centerfire primers are the two categories of primer ignition systems for cartridges. But before you can tell them apart, you’ve got to the know the parts.
Parts of a Cartridge
A standard cartridge, or round, consists of four parts—the bullet, propellant, primer, and a case. Keep in mind that a “bullet” means just the projectile, not the entire cartridge. These components are all present regardless if a round is rimfire or centerfire.
In the grand scheme of things, all bullets work the same…the firing pin of the gun hits the primer which creates a tiny explosion. That tiny explosion sets off the gunpowder which forces the bullet itself forward and out of your gun through the barrel. More on how a gun works.
The difference between the rimfire and the centerfire is in where that primer is located.
Centerfire vs Rimfire Appearance
The easiest way to tell between them is to see if you can see a circular primer in the center at the base of the casing. *Gasp* Primer in the center = centerfire! If you see a smaller cartridge with no overt primer, it is likely rimfire.
Different Ignition Systems
You can see that the names really make sense when you look at the ignition systems. Centerfires are ignited by a firing pin hitting the center primer, while rimfires have firing pins that hit the rim of the cartridge.
You can see the firing pin marks on the spent brass above. Again, centerfires hit the center primer while rimfires hit the rim.
Common Types of Rimfire Ammo
Rimfire ammo is limited to smaller calibers since the cartridge walls need to be thin enough to be able to be crushed by the firing pin and ignite the primer. The downside? Well, the nature of the casing means it’s pretty much limited to small calibers. You have to have some relatively flimsy brass to handle the rimfire set up. As a result, the powder necessary to propel a larger bullet would blow the brass apart.
The most common type is the .22LR (long rifle).
- .22 Short: Used in some revolvers, not too popular
- .22 Long Rifle (LR): Most popular round in the world and the starting point for many shooters.
- .22 WMR: Winchester Magnum Rimfire. Used to hunt vermin and is between the .22LR and the .223 centerfire round.
- .17 HM2: Hornady Mach 2: Higher power than the .22LR but smaller. Didn’t really take off. Smaller brother of the HMR.
- .17 HMR: Hornady Magnum Rimfire. Newer round that is flatter shooting and more powerful than the .22LR
Pros of Rimfire Ammo
- Cheap: .22LR can be found for 5-10 cents while the AR-15 .223 round is between 30-40 cents each. It is so cheap since it is easier to manufacture a thin walled case with a flattened primer at the bottom. The downside? Well, the nature of the casing means it’s pretty much limited to small calibers. You have to have some relatively flimsy brass to handle the rimfire set up. As a result, the powder necessary to propel a larger bullet would blow the brass apart. However, there has been some hoarding in the last few years which makes it a little difficult to find in stores and online.
- Low Recoil: The .17 and .22 caliber bullets and small amount of gunpowder make for extremely low recoiling firearms. Perfect for the beginner or for training.
Cons of Rimfire Ammo
- Not Reloadable: The primer is inside the bottom of the case and so cannot be reloaded like centerfire rounds. But rimfire ammo is so cheap comparatively that it doesn’t matter.
- Reliability Issues: Even with top shelf ammo like CCI, I still manage to get 1-2% failures to fire (FTF) in my 10/22 rifle. This is because in manufacturing, the primer compound is “spun” at the bottom of the casing and sometimes does not make full contact with the entire rim. Rimfire is great for range plinking and varmint hunting, but I would not trust it for personal defense.
- Small Calibers: Again, because of its design, rimfire is stuck with small calibers. There’s some exotic larger caliber rimfires out there but they are very rare.