I tried out many of the suggestions. So far,
4 different magazines-- same problem
extreme amount of lube -- less frequency, but still same problem
one round in mag -- Wolf HP 55gr, consistently not enough gas to lock back bolt
-- PMC Bronze fmj 55gr, locks bolt back everytime
-- AE XM193 fmj 55gr, locks bolt back everytime
Before changing the upper reciever the gun would shoot the Wolf HP 55gr but not the Wolf fmj 55gr. I also chrono'ed the HP wolf at about 2700 feet per second. The 5.56 XM193 chrono'ed at about 3200 feet per second.
Small amount of leakage at gas block
Also tried bolt out of M4, same problem
And M4 lower, same problem
The most interesting finding was that I had the same jam once while hand chambering a round. That was an insert magazine on closed bolt and try to rip the charging handle off to chamber the round.
I don't see evidence of bullets catching on ledge between the m4 style reciever and the rifle style barrel extension. I am still weighing my options on correcting that issue.
How about that for a jumbled mess of info!
Rob