Rim Lock
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).
Contents
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.
- Angled magazine & feed lips: Many modern guns chambered in semi-rimmed cartridges, such as the Kel-Tec P32 use precisely shaped magazine & feed lips and such positioning that it tries to make sure a cartridge is always in front of the next, never allowing the next cartridge's rim to hang up the one before it. However, with loads such as shorter hollow points, there is still sufficient space in the magazine for cartridges to move back and forth, allowing potential rim lock. This brings in the idea of a "flyer wire", a DIY magazine mod for such pistols.
- Flyer Wire: The idea of a flyer wire is to install a steel wire through the rear part of the magazine, such that cartridges are always forced into a forward position and prevented from shifting. If rounds can no longer shift, they can no longer jump over the rim of the next cartridge, preventing rim lock. This mod appears to be named after the user "Flyer" from the Kel-Tec Owner's Group (KTOG) forum, who came up with the idea. It remains a popular and cheap modification for the P32 and similar handguns. There is a guide to installing it into P32 magazines at this link.
Other Articles
External references
Reddit thread addressing Mosin-Nagant Rim Lock
Gunboards thread explaining the workings of the Mosin-Nagant's interruptor arm.
