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ARMAG

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Rimmed case

A cartridge case with a flange at its base wider than the body, used to headspace the round and to grip it for extraction.

Also called Rim, Rim lock

The oldest arrangement and the simplest to manufacture: the case has a lip at the back, the chamber has a step, and the round can go no further forward. The lip also gives an extractor something unambiguous to grip.

What it costs is stacking. Put rimmed cartridges in a box magazine and the rim of a lower round can sit in front of the rim of the one above, so the upper round cannot slide forward. That is rim lock, and it is why the Lee-Enfield’s ten rounds must be loaded with each rim ahead of the one below, why the Mosin-Nagant needs Nagant’s interrupter to hold the second round back, and why a belt-fed gun firing 7.62×54mmR has to pull each round backwards out of the belt before chambering it.

Rimless cases headspace on a shoulder or a case mouth instead and stack without complaint, which is why almost every cartridge designed for a box magazine since about 1890 is rimless.