Quoted:
I'm no expert, but I would say the 'pinging' on the back side of the locking lugs occurs on bolt closing, not opening.
When the AR bolt closes, they 'bounce' off the breach-face and start to turn as the lock up. Since the bolt is not under spring pressure (only the carrier is) the bolt is free to bounce back and ding into the barrel lugs before the carrier continues to close and forces the completion of the turn. Not sure what long term problems this may cause, if any.
Since the 308 bolt is considerably heavier than the 223 bolt, the 'bounce' would certainly be more pronounced.
Not saying the problem is 'normal', but I'm curious to learn if the issue becomes pronounced or effects reliability in the long run.
I agree. The damaged lugs on the OP's bolt is caused by bolt bounce when closing. All ones needs to do is look at how the bolt rotates in to the barrel extension as it closes.
I can't say about the BM RE and buffer, but I do know that DPMS RE's (consolable) are AR15 length, use a modified (shortened) AR15 buffer. Where as ArmaLite, KAC, Noveske, LMT, and CMMG all use a RE (consolable) that is a half inch longer to compensate for the longer .308 bolt.
Just over 1K in my Noveske N6 and the back of the lugs are just starting to wear the finish.