Rebated-rim cartridges have a rim that is significantly smaller in diameter than the base of the case, serving only for extraction.
Functionally the same as a rimless case, the rebated rim allows a gun to be easily converted to fire a larger-than-normal cartridge, as most of a firearm's loading and extraction mechanism does not need to be altered as long as the rim size is preserved.
[citation needed] Rebated-rim cartridges are known to be used on firearms using advanced primer ignition (API blowback) for its operation, notably autocannons such as the 20 mm Becker/Oerlikon series.
[1] API blowback firearms have straight-sided chamber walls as possible (and sometimes slightly bottlenecked) to contain the cartridge and allow a deeper chambering.
[citation needed] The bolt face has the same diameter as the rebated-rim cartridge as it follows along with the extraction claw into the chambering.