I'd call Bushmaster, also check their web site for tech advice & FAQ's
I don't know a lot about AR-10's, but in AR15's......
In-spec headspace is super-important. Is it possible yours is out of whack? Maybe it's time to have a gunsmith look at it.
Are you sure the bolt is closing all the way?
[b]"used several brands of brass(4)"[/b]
are these reloads? too hot?
I would also wonder about the gas tube & key.
Whatever the cause, after several shots like this, I would also want to make sure the carrier and all parts are in new condition & if in doubt, get them replaced.
If you don't have go, no-go, & field guages, take it to a gunsmith who does.
The extractor spring is only supposed to go in one way.
Did you examine the brass for other signs of trouble (bulges, primer issues etc)?
[red]PRK