I bought a Dec 1941 Remington 511 for effectively nothing. Seller disclosed the bolt would easily ride out after the trigger had been pulled. Apparently this is a common issue, as the sear works as a bolt stop and can get buggered over. I bought a replacement sear from Numrich. Its about 1/4"...
That sounds about right. I only have one mag, but went and ordered a few more, along with the clip that holds the slide stop. I think that if it does not have enough tension, it will float that end of the slide stop back and forth, allowing the mag follower to miss it on the way up.
I have shot a surplus Polish Tok a few times and always eventually run into the same issue. It seems the slide stop part that interacts with the magazine seems to get below the follower in the mag, and then causes everything to lock up. The slide will not return to battery, the mag won't drop...