A little late, but sorry about you friend.
I can testify to your "... watch out for excessive chamber pressure" comment. Many, many years ago when I was curious about the MMJ 5.7, I was given a barrel in "....22 - Carbine, might be a Johnson Spitfire" where the gas chamber had "blown out". As I recollect, the gas cylinder had cracked and the piston was frozen in place. The old feller (I'm probably older now than he was at the time), had put the barrel on an action of some sort and he says he'd been shooting it - AS IS!. He gave me the barrel after he had replaced it with a normal barrel. I still have that barrel around somewhere, this thread made me think about it so now I have to go dig around and find it...
