Well, I'll go ahead and tell ya what I'm trying to do. I'v got a L1A1 FH that I want to put on the barrel of my AMD65, to bring it up to legal length. the id of the FH is .590, the od of the threaded part of the barrel is .490. What I am thinking is to drill out the id of some 4130 alloy tube to 13mm, tap it with a 14mm LH tap, then turn down the od of the tube to approx .590, or just enough to slipfit it to the FH, then silver solder or weld them together. then screw it onto the barrel. The FH brings the barreel to +16 inches, and it has a bayonet lug for pissing off the liberals.