Every 9mm AR I've ever seen used a bolt that was non-rotating/locking, but still essentially fully surrounded the case head. If fired cases are notably bulged relative to unfired ones, then you have a timing problem. Either pressure is still high after the bolt moves back, or the gun is firing before it goes into battery.
Since the rounds gauge oversized to start, I would start by removing the bolt and checking if 100% of rounds will "plunk" in the chamber. If not, there's a good chance that you've got rounds firing out of battery. if they all drop in with just gravity, it's likely it's unlocking too soon.
