Rim Lock

An illustration describing rim lock on rimmed and semi-rimmed firearm cartridges. The left side shows proper stacking of rounds, allowing for reliable feeding. The right side shows rim lock, where the rim of the first round has become obstructed by the rim of the following round, causing a jam.

Rim lock (also written as "rimlock") is a phenomenon that occurs in magazine-fed guns that fire rimmed or semi-rimmed cartridges. Some examples of such cartridges include .22 Long Rifle, .22 Magnum, .357 Magnum (rimmed) as well as .32 ACP and .38 Super (semi-rimmed).

The issue is that in a magazine-fed firearm, particularly when involving a detachable magazine, the top most round in the magazine can sometimes slip backward behind the rim of the next round, causing the gun to seize up and be unable to feed the round from the magazine.