Finally got motivated and looked at the bolt on the 6.8 build that's been problematic. First thing of note when removing the BCG was the charging handle not wanting to come loose. This must be what caused the problem I thought was the cam pin. Like a dummy I torqued it down (due to coming loose) and forgotten about it. Got an email in to Gibbz asking for proper torque.
Hopefully it's another tolerance/sizing issue like with the skull build. I just dremeled out more of the lip the CH latches onto with that one. I was much more aggressive so hopefully that's that. If not, there's plenty of room to go. Or maybe I stick a milspec CH on there, see if it stays put, and call it a day. But I can torque it down so it stays put, it's just a question of that potentially gouging out the receiver.
So since it was otherwise working, maybe it was just break-in. Here's the bolt after 250 rounds.
I thought part of why the handguard nut came loose might b the gun recoil producing some torque. The muzzle device appears vertical. I'll take a wrench out next time and use slow-mo video to tweak it. The thing was jumping to my left.