Definition:

A cartridge is called rimless if its rim is very close to the same diameter as the adjacent case body, but is no larger than the body at that point.

Although the cartridge truly does have a rim, the fact that the rim doesn't protrude laterally past the side walls of the largest portion of the case body means that the rim's only purpose is to provide purchase for the extractor.

In a rimless cartridge, the rim does not serve to headspace the cartridge.

Rimless cases were designed to improve feeding in repeating arms; cases with rims that protrude out beyond the case body tend to cause feeding problems far more often than rimless ones.

- Russ Chastain

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