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Cartridge Cautions: Firing Cartridges Based on the Same Case

For safety's sake, factory ammunition cartridges have only one set of specifications. But cartridges based on the same case can often be fired from a chamber for the parent cartridge. Craig Boddington explains.

Cartridge Cautions: Firing Cartridges Based on the Same Case

In designing cartridges, manufacturers go to great lengths to preclude accidental interchangeability. However, many cartridges are similar in appearance and dimensions. On the range, it’s wise to have only one cartridge/caliber on the bench at a time.

By long-established conventions, a factory cartridge has just one official name, and one set of case dimensions, pressure specifications and all the rest. In America, the rules are set by SAAMI. In Europe, it’s CIP. Both try to ensure that a new cartridge can’t be unsafely fired in a chamber cut to any existing factory cartridge.

Note the word “unsafely.” Cartridges based on the same case can often be chambered and fired in a chamber for the parent cartridge, provided the daddy has a larger-diameter bullet.

For instance, a 6mm Creedmoor will chamber and fire in a 6.5mm Creedmoor. While filming a TV show, one of my colleagues couldn’t get a 6.5 Creedmoor rifle on paper. It took several shots to figure out he was shooting 6mm Creedmoor.

Similarly, various cartridges based on the .30-06 case with bullets of smaller diameter, such as the .25-06 Rem. and .270 Win., may chamber and fire in a .30-06. Likewise, with the .308 family, the .260 and 7mm-08 Rem. and the Creedmoors may chamber and fire in a .308.

The smaller-diameter bullet just rattles down the bore. With less containment of expanding gases, there’s little danger and no accuracy, but it’s still a dumb mistake. On the bench, one rifle and one cartridge at a time, please. If someone hands you an unfamiliar rifle, never be shy about inspecting barrel markings and cartridge headstamps.

Case dimensions for the .360 BHMR
This is the mechanical drawing for Remington’s recent .360 Buckhammer cartridge. All cartridges have just one set of official dimensions, standardized and accepted by SAAMI in the United States, CIP in Europe.

Years ago, I was hunting muntjacs in England, borrowing the deerstalker’s rifle. The first morning it was raining cats and dogs. This led to the cardinal sin: We didn’t check zero. When the rain let up deer were moving like crazy. We headed out, and he handed me his .243. I missed two muntjacs, making for a long and embarrassing afternoon.

That evening, I was soothing my ego with Boddington’s Pub Ale (almost mine; my great-grandfather left the brewery in 1860 and came to America). I reached into my pocket and set an empty cartridge case on the table. Short-necked and blown out, it didn’t look like any .243 case I’d ever seen. I grabbed my reading glasses and looked at the base. It was a .22-250, not a .243. Our stalker had two similar rifles, one .22-250, one .243. No wonder I couldn’t hit anything.

Although it was stupid, no real harm was done. Base and rim diameter were the same, so the extractor held the cartridge as it should. It chambered and fired, and the bullet bounced off into the English countryside.

The danger comes when trying to swage a too-large bullet down a bore. Depending on how well the extractor grips the case, there are many examples where it is theoretically possible to chamber and fire a shorter cartridge with a larger-diameter bullet. Which goes back to the guidance that you should always check the roll-mark stamping on the rifle, and check the ammunition box and headstamp.




There is almost no useful centerfire rifle cartridge interchange—and none is recommended. With few exceptions, a rifle cartridge has just one official name and, always, just one set of specifications. Engineers manipulate the specs to preclude dangerous interchange, usually by neck length. For instance, the .270 Win. and .280 Rem. have the same .30-06 parent case and same case length of 2.540 inches. However, the .280 Rem. has a longer case body and a shorter neck—specifically to preclude it from chambering in a .270 Win. rifle.

With numerous families of cartridges based on the same parent case, appearances can be very similar. A few years ago, I wanted to check zero on a .270 WSM, using an old box of factory ammo. On the third shot there was sharp recoil, smoke eddying out of the receiver, lots of pain. The case head had ruptured, and I was bleeding from a ring of shrapnel around my shooting glasses. Fortunately, I was wearing good glasses, another unarguable must.

I had fired a cartridge marked 7mm WSM in a .270 WSM chamber, putting a .284-inch bullet down a .277-inch barrel at high velocity and pressure. Luckily, there was no permanent damage.

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I’ve never owned or tested a 7mm WSM, so how did that cartridge get in that box? I was on a hunt in Utah when the .270 and 7mm WSMs were unveiled. At that time, both were experimental. Prior to release, the 7mm WSM case was modified slightly—with shorter neck and longer case body—to preclude chambering in a .270 WSM.

It gets more complicated. September 11 happened during that hunt, and things were confusing. With no planes flying, I was in a fizz to get a rental car and report in at Camp Pendleton just in case they needed me. We rushed to clean things up, and I took that box of “.270 WSM” ammo with me.

I stowed it away and didn’t open it for nearly 20 years. In our haste, somebody—probably me—put one of those still-experimental 7mm WSM cartridges in that box.

Headstamps exist for good reason, and they are there to be read. However, there are instances of multiple names for a cartridge. Winchester, for example, at times used WCF (Winchester Center Fire), other times not. The .30 WCF often seen on early Winchester rifles is the good old .30-30 Win.

Perhaps the most famous dual-name cartridge is the 7x57 Mauser aka .275 Rigby. In 1907, John Rigby was Mauser’s exclusive agent in the United Kingdom. With Mauser’s approval, they simply renamed the cartridge for the British market. Case dimensions are identical, so it’s not even a matter of interchangeability; they are the same cartridge.

In modern times, the primary example is the .280 Rem./7mm Express Rem. Hoping to bolster sales with a catchier name, in 1978 the .280 was briefly renamed 7mm-06 Rem. I’ve heard of no ammo marked this way, but occasionally a factory rifle so roll-marked will surface.

Within months the company changed it to 7mm Express Rem. Same old .280, but now it got confused with the 7mm Rem. Mag. In some 7mm Rem. Mag. rifles, a .280 (or 7mm Express Rem.) cartridge might be chambered and fired, almost certainly causing a detonation. After a couple of years of confusion, Remington quietly renamed it back to .280 Rem.

The rule is simple: When it comes to rifles, roll marks and headstamps need to match, and we need to check both. If there’s any confusion, do some research. Factory cartridges may become obscure, but the specifications are out there and do not change. Wildcats and proprietaries are another subject. Maybe I’ll save that for another story.

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