do not use live rounds for testing and adjustment

Rim lock is a frequent complaint with Mosin rifles. People attribute rim lock to all sorts of reasons from "poor ammunition" to "Soviet" quality. The most probable explanation for the high occurrence of rim lock is the somewhat random assembly process followed during refurbishment. Possible explanations are many but the purpose of this thread is to show how the design specifically avoids rim lock and what to do to fix your rifle if it does occur.Rim lock occurs when the rim of the top round is behind (rearward) of the rim of the second round. When you try to close the bolt you are trying to shove the second round through the magazine. When this happens the top round can be seen trying to plunge downward. If the interrupter is working properly this condition can be corrected by pushing down firmly on the top round. If the interrupter is not working correctly you have to dump the magazine. Either way it is frustrating and a distraction.First an explanation in words and then some pictures. My pictures show dummy ammunition;. If you don't have snap caps or dummies, you can use spent cases (but only from the rifle they were fired in).The key to preventing rim lock is the. This is a spring loaded small finger that holds theround below the first in the magazine. When the bolt is open, the interrupter holds the second round securely below the first so the rims cannot interfere with each other. When the bolt is closed the ejector is pushed outwards which disengages the interrupter and allows the rounds to rise in the magazine under pressure from the floor plate spring.Picture 1 shows a magazine with five rounds (dummy) loaded. The top round is held in the feed channel by the ejector (under spring pressure) and the small detail on the receiver.Picture 2 shows the second round (first round manually removed) which is held by the interrupter. The interrupter is holding #2 below the point when its rim can overlap with round 1.Picture 3 shows the receiver and magazine assembly without the stock. Remove the stock and re-assemble the parts (snug the screws only) and clamp in a vise so that you can see the operation of the interrupter. With the bolt closed the ejector and interrupter are at full inward travel. The thickness of the ejector tips dictates how far the interrupter/ejector assembly will travel inward (more about this later).Picture 4 shows a detail of the interrupter. With the bolt open, the interrupter is firmly engaged on the 2nd round holding it below the point of interference. This is the key to preventing rim lock; the interrupter must firmly hold round #2 with the bolt open.Picture 5 shows the bolt closed. The interrupter/ejector assembly is pushed outward by a detail on the bolt body.Picture 6 shows a detail of the interrupter disengaged allowing round #2 to move freely upwards (round 1 is in the chamber).Picture 7 shows as received (i.e. not modified by me) ejectors that differ in tip dimensions. Filing material from the tips of the ejector is the method used to adjust the interrupter to ensure that it firmly grips the second round.If the interrupter does not firmly engage the second round, remove a small amount of material at a time from the ejector tips. You can (carefully) remove the ejector without removing the spring (small screwdriver and pliers) to make these adjustments. Sharpening the engage surface of the interrupter will improve the grip. You can bend or twist the spring to get more engagement but risks breakage. (some body will post "I bent it no problem" but I used a method for optimizing trigger springs that worked 8 times then I broke 2 in a row so beware).Hope this is helpful to some. I will expand on anything that isn't clear.