(1) Over-bulging of the crimp on the end of the shell.
(2) Slight corrosion on the head of the shell.
These are the only two things that have caused my auto's to hang up. This can get a little bothersome while shooting sporting clays.
I almost forgot, I used to have a 1100 classic trap and shot doubles with it a couple of times. The pin that was supposed to deflect the hull down hung up the empty hull every once in a while. I filed the pin down a little and ended that problem.
I have 2 390s and have never had a problem using steel based Rem Gun Club or Estate shells. It's certainly not something that 390s are known for; Ljutics for example are well known for this problem. I think bridgetoofar has it right, though. Chamber sizes will vary, and if you're having a problem best let a competent gunsmith look at it. If you're just concerned that it might be a problem, I wouldn't worry about it. James Fawcett
Where you WILL run into trouble is by picking up discarded shells and then not resizing them when you reload. You're bound to run across some that were shot in a gun with a larger chamber diameter than yours and that's a formula for a jam. Any hull used in auto-loaders should be resized....breakemall....Bob Dodd