The puzzling thing to me is that it's kind of a random thing, only affecting 1 out of 10 cases. But Unclenick most likely revealed the culprit as the auto rifle that fired the cases first. I don't have any experience with them.
I do know each rifle is very unique though. You might have to do l ike I do and keep the brass separate for the various rifles of the same caliber. I have 2 different 7 mm mag rifles. One of them, a Browning BLR behaves like about every other rifle I load for in terms of sizing brass. No problems at all. The other is a custom job with the chamber custom reamed. When I compare the fired brass to brand new brass, it is only about 0.002" longer to the shoulder datum after I fire it. When I run it through the FL resizing die, it is extremely difficult to push the last 1/4" or so. The die is having to resize the body of the case a lot. But then the die doesn't push the shoulder back at all, no matter how much I adjust it down. I have to insert a 0.005" feeler gauge beneath the case head, between it and the shellholder to get the shoulder bumped back by about 0.002". That rifle and its needs probably scream for neck sizing only. The main thing though is that I don't mix the brass from the 2 different rifles. That would complicate things even more.