My guess is that they might have checked headspace while the barrel wasn't installed? Inmost cases there's nothing wrong with that process. I've had a few that would headspace uninstalled, but headspace was incorrect after installation using the same bolt and BCG. Too much headspace is worse than too little (or so I've been told) , but can't confirm. Not sure if the key or the bc was the issue. How did the firing pin look? Was it normal or was the tip damaged? I wonder if the pin ruptured the primer. I have had a few do this. Those weren't factory built guns that I've had issues with. Building 50+ ARs has been a good learning lesson. I'm no expert, just experienced. Whether it be 5.56 in NATO or Wylde and 7.62NATO and 308Win from pistol too rifle length gas systems are the easiest/best working gas systems. When you have offsprings in either platform it can get tricky on cycling without case, primer, fte, ftf or bolt holding open on last round. If you were too close the gas block you still shouldn't have those issues. Regardless of weak spring, bad buffer or gas rings you still shouldn't have this problem. As mentioned by others if ammo is fine (maybe varify with other guns if same cartridge) it should be headspacing? With that being said I have had blown primers with Hornady 6.5CM factory ammo in a AR10 platform. I've used the same ammo in other AR10s from 18" to 24" barrels with no primer problems. From aero precision with rifle length gas system all functioned properly. Other mfgs with standard and +2" gas system primers would blow. So I wouldn't say it's ammo related, but not ruled out?