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 an issue that can occur in magazine-fed guns that fire rimmed or semi-rimmed cartridges. Examples of such cartridges include .22 Long Rifle, .22 Magnum, .357 Magnum, 7.62x54r (all rimmed) as well as .32 ACP and .38 Super (both semi-rimmed).

What causes rim lock?

The primary issue is that in a magazine-fed firearm, particularly when involving a detachable magazine or internal box magazine, the top most round in the magazine can sometimes slip backwards behind the rim of the next round, causing the gun to seize up and be unable to feed the round from the magazine. Alternatively, the user can easily load a round in this way by accident. This is not really an issue in firearms with a tube magazine because the rims have no way of grabbing each other and locking up.

Preventing Rim Lock

Careful Loading

In many firearms, rim lock is relatively easy to avoid. By taking care to make sure that each round's rim is sitting in front of the previous one, this should eliminate most instances of rim lock. However, it may be the case that some magazines are not designed to properly allow for this but give just enough room for cartridges to lock together and still cause issues. Overall, paying attention during loading can resolve many instances of rimlock.

Mechanical Prevention

Many designs have implemented some features to address this serious issue.

• Mosin-Nagant: Implements an interruptor, a small arm that swings into the magazine when the bolt is open in order to keep the 2nd round pressed downward. From this Reddit thread, many agree that a faulty interruptor is a common cause of feeding failures or double feeds in Mosin-Nagant rifles. Also, improper tension, or being able to catch on the stock when it shouldn't, are common causes of rounds failing to sequence correctly. Users say that simply adjusting or replacing the spring can often fix these issues.