For some time now I have noticed a common way of thinking. If there is a problem and brass case is used, its the gun. Same problem and steel case is used, its the case. I have had case separations on steel and brass case. Only 2 in 40 years... thats a crazy low amount. I had 2.5 times that amount from the last magazine I bought through my Chauchat. 8mm Lebel Prvi and yes the barrel is going to back for some more work. There have been many out of batteries with the semi MG42s. Most seem to be on the first round after loading the belt. This is blamed on the charging handle slowing the bolt and not having it seat fully before the gun is fired. The main problem is that there is nothing to stop the hammer from hitting the firing pin if the bolt is not fully closed. This is why I bought Brian's new made bolt. I don't know if it fully prevents this but I believe it should cut down on this issue. However from what you are saying, that the second round had the case separation... that makes no sense to me from a Poly stand point. If the round that you left in had heated up, melted the coating and cooled... making the poly somewhat solid again... then maybe I could see it (but not really). However the second round would not have that issue and the barrel would have had time to cool down. This is the same issue that came up with the Lacquer coated stuff... rumor mill got going the cases were sticking from the lacquer. So they switch to Poly. Now people are blaming poly. However I am fairly convinced its not the coating doing this! There are many YT videos on it. If anything its really an issue with the extra carbon build up from the steel case not expanding as well.You opinion is noted. The weapon was brand new, I put 4, maybe 5 belts of the Romy 8 mm through it and it ran like a Swiss watch. I stopped shooting to deal with something else, with a belt in it and with a round in the chamber. I started firing...bang, bang, boom. The consensus was that a case head separated either on the second round. A least one bullet passed through through the case “ironing” the neck out to point a ruptured case extractor had nothing to grab, hence the finishing reamer being needed to remove it.... Both we’re running polymer coated cases. ...The only common factor was both times the case was polymer coated. I’m using up what I have and not buying anymore of it.
The fact is that steel case has been used by several militaries starting before almost everyone here was born. They did not report these issues in general. That is not to say there are not bad steel cases out there, just like there are bad brass case out there. However the Russian's were not making millions of steel case in the cold war because they were ok with a common level of case separations due to the lacquer melting in the chamber.
I will also add that just because there was a case separation... which does happen with both steel and brass case... that should not result in an out of battery in a correctly designed firearm. Now I know these Semi MG42s are not designed as safely. However I think it makes a good case for spending the extra money for Brian's MG42 new made bolt carrier as it appears to block the hammer from swinging all the way home if the bolt carrier its not in the correct position forward.