I have one of these that had that same problem. I found a short spacer made of wood stuck in the carrier apparently by someone shooting .22 LR in it. Doesn't seem to have hurt the gun, but the heeled LR bullets tend to expand and jam in the throat just enough to raise pressure to the point of piercing the rim, so I recommend against that substitution. The rim has no surround in mine.
Anyway, take it down and look for something that stops the cartridge going all the way to the rear of the carrier.
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