Joe - THANK YOU for your help (and patience with me). If that is the EASY way to check headspacing, I'd hate to see the hard way. That operation just assumes I have .0003 shim stock and a feeler gauge just laying around. (as I said, I'm not a metal worker or gunsmith.) I understand headspacing as it applies to my 1919a4 for example. The distance from the end of the barrel to the bolt face. However, when the barrel has huge cut outs on it (exactly where the cases are blowing out) I don't see you can ever make the headspacing tight enough to matter.) Again, I don't know. I followed your directions as best I could and took pics. Not sure it helps. (see attached) I suspect that I can send it to someone to get the headspacing checked.

Question: what are the "locking shoulders" everyone keeps talking about? And what is movable in the receiver to change the headspacing anyway?
Last observation, your second hypothesis (i.e. that the gun is out of timing- makes logical sense as it does appear that the bolt may be moving back before the pressure has dropped to a safe level.) Can this happen only once after 60 rounds or so? (B/C that is when it happens). What can we do if the rifle is mistimed?

Again, thanks for all the help. I very much appreciate it.