Another shell exploded as happened before - this time with brand new ammo.
I agree with you about returning the gun. The problem is, I bought it at a PRO show from another attendee. I don't have any way of finding him.
After the second shell blew up, I showed it to an M1 expert a friend knows. He wasn't concerned about the reassembly of the barrel. I'd been very careful when removing it. The root cause of all my problems is actually the receiver. There's been a bit of machining done to it, and even the expert didn't know why it would have been done. The bolt is loose because of these modifications, and causes an unusually high amount of head space. You can't see all of it in the picture, but notice the small groove cut near the back of the left side of where the bolt engages. It's even worse on the right side (not visible).
The expert told me that it was unlikely that even he would have noticed this if he had bought the gun (although he wouldn't have bought a Nat'l Ord.) He suggested I take it to another show and dump it off on somebody else. Unfortunately, my conscience won't let me do this - somebody could get hurt firing this gun. I will either have someone do some welding, and try to stone it down to the correct specs, or find a new receiver.
Speaking of receivers, anybody have one they want to sell?

I'd prefer a GI this time, but the actual maker isn't that critical. There's an interesting medley of parts on this gun : Winchester barrel, Springfield armory M2 stock, an Inland barrel band and M2 slide, a Saginaw trigger group assembly, actual trigger is Rockola. All of these are in excellent condition (many compliments on the stock). There's some markings underneath the rear sight that may indicate that the receiver might have been made by someone else, and Nat'l Ordinance restamped it. I'll have to remove the sight to see what exactly it says (it this hard to do?). It has an M2 magazine catch, and I don't understand what the difference is.
The positive side to all this- I've learned alot about M1's that I wouldn't have learned if the thing worked right to begin with. Oh well - live and learn.