Canada · ammunition study notes

Rimfire vs Centrefire Ammunition

By Hunters Guide. Checked against SAAMI technical publications and RCMP course material on September 18, 2026.

Rimfire cartridges carry their priming compound inside the hollow rim of the case. Centrefire cartridges carry it in a small replaceable cup set into the centre of the case head. Almost every other difference between the two follows from that one.

Where the priming compound sits

Priming compound is the impact-sensitive chemical that ignites the powder charge. Where the manufacturer puts it is what names a cartridge rimfire or centrefire.

In a rimfire case, that compound is spun out into the hollow fold of the rim during manufacture. It sits in a ring all the way around the base. In a centrefire case, it is packed into a metal cup with a tiny anvil inside. That cup is pressed into a pocket dead centre in the case head.

Which changes what the firing pin has to do. A rimfire pin strikes the edge of the base and pinches the rim between the pin and the steel at the rear of the barrel. The rim crushes, the compound detonates, the powder lights.

A centrefire pin strikes the middle, dents the primer cup, and crushes the compound between the cup and the anvil behind it. Same outcome, different geometry.

And that geometry has a consequence people often miss. For a rimfire to work at all, the rim has to be thin enough to squash. Thin rim, thin case head, low pressure ceiling.

It is why rimfire lives at the .22 end of the world and always will. Centrefire cases, with a thick solid head, hold pressures many times higher. It is also why only one of the two can be reloaded.

Why centrefire cases reload and rimfire cases do not

Reloading is the practice of rebuilding a fired case into a live cartridge with a new primer, fresh powder and a new projectile. Whether a case can take that depends entirely on whether the primer is a separate part.

On a centrefire case it is. The spent primer punches out of its pocket and a fresh one presses in. The brass case itself can be resized, trimmed and reused a number of times before it fatigues. That is the whole reason a handloading bench exists.

On a rimfire case, there is nothing to replace. The priming compound was distributed inside the rim when the case was formed, and the firing pin permanently deformed the rim getting at it. No pocket, no cup, no anvil, nothing to swap.

So rimfire ammunition is a consumable in a way centrefire is not. Spent rimfire brass goes in the scrap bucket, not back to the press.

What does the headstamp on a cartridge tell you?

The headstamp is the set of markings pressed into the base of a cartridge case. It is identification stamped on the ammunition itself, not printed on a box that can be swapped. SAAMI defines it as "Numerals, letters and symbols (or combinations) stamped into the head of a cartridge case or shotshell to identify the manufacturer, caliber or Gauge, and other additional information".

Here is why that matters more than anything else on this page. The firearm carries its own marking saying what it is chambered for. SAAMI puts it plainly: markings "which indicate the appropriate ammunition to be used in a firearm are usually found on the firearm's barrel but may alternately appear on the frame or receiver".

Those two markings, the one on the round and the one on the gun, have to agree exactly, not roughly.

One honest caveat. Rimfire cases frequently carry no calibre marking at all, just a maker's letter or nothing. On rimfire, the original packaging is your reference instead. If both the headstamp and the box are gone, you have an unidentified round, and an unidentified round does not get chambered.

Does a cartridge fitting the chamber mean it is safe to fire?

A cartridge that chambers is a cartridge that physically fits, and physical fit is not a safety clearance. This is the single most expensive misunderstanding in ammunition handling, and it catches experienced people, not just new ones.

SAAMI states it directly: just because a round fits into a chamber, barrel or action, "this does not necessarily mean that it is safe to use that ammunition in that firearm".

A shorter or narrower cartridge can slide into a chamber cut for something else and leave the case unsupported. Or it sits correctly and simply generates pressure the firearm was never built to contain. The action closes. The trigger works. The barrel is the part that finds out.

Which is why the rule is mechanical rather than a judgement call: read the marking on the barrel, read the headstamp, and load only when they match. No interpretation, no close enough. If you want to see how this gets examined, the CFSC practice test covers ammunition selection alongside the rest of the course material.

The four parts of a centrefire cartridge

A centrefire cartridge is built from four parts: the case, the primer, the powder, and the bullet. Learn them in that order, because it is the order the energy travels in.

Note the wording on the last one. One bullet. A rifle or handgun cartridge sends a single projectile, which is exactly where a shotshell parts company.

Why a shotshell has five parts instead of four

A shotshell is a shotgun cartridge built from five parts: the case, the primer, the powder, the wad, and the shot. Two of those are the difference. The single bullet is replaced by a charge of many small pellets. A component called the wad is added between the powder and the shot.

The wad has three jobs stacked into one plastic part. It seals the bore so gas pushes the shot rather than blowing past it. It cushions the pellets so they deform less as they accelerate.

And it keeps the shot column gathered until it clears the muzzle, then peels away and drops off downrange. You have probably picked one up in a field without knowing what it was.

Shotshells can also be loaded with a single slug instead of shot. Worth knowing, so the five-part model does not lock in your head as a rule with no exceptions. If you want to drill the components until they are automatic, our ammunition practice questions work through them one at a time.

Questions about rimfire and centrefire

Is .22 LR rimfire or centrefire?

Rimfire. The .22 Long Rifle is the rimfire cartridge almost everyone meets first, along with .22 Short, .22 Long and .22 WMR. Look at the base of a fired case and you will see a dent on the edge of the rim rather than in the middle.

Can rimfire cases be reloaded at home?

Generally no. There is no separate primer to replace, because the priming compound was spread inside the rim when the case was made, and the firing pin deforms the rim itself. Rimfire cases are consumable. Centrefire cases are not.

Which parts of a shotshell leave the barrel?

The shot and the wad both leave the muzzle, along with the powder gases. The case stays in the chamber and gets ejected. That is worth knowing because a wad on the ground downrange is evidence of a shot, and a wad stuck in a barrel is an obstruction.

What should you do if a headstamp is unreadable?

Do not fire it. Go back to the original packaging to identify what it is, and if that is gone or you are still unsure, have a qualified person identify the round before it goes anywhere near a chamber. Rimfire cases in particular often carry no calibre marking at all.

Is rimfire ammunition less powerful than centrefire?

As a rule, yes. The rim has to stay thin enough for a firing pin to crush it, which caps the pressure the case can safely contain. Centrefire cases use a thick, solid head and a separate primer, so they can run at far higher pressures.