Brit Plumber,

Okay...thanks for the guidance. On my kit gun, the ports on both the tube and the magazine housing were made with that taper to the front (smaller front than rear) I will do the same with my build. They both seemed to match up pretty well. I'll try to do the same.

If they were rivets, they were filed flush to the contour of the tube. It seems like the diameter of the top is larger than that which shows in the inside of the nut. They look like pins to me from the inside, but then with the larger upper diameter i guess flat head rivet might be correct.. I am thinking about drilling three holes into the end of the outer tube only, inserting the barrel nut and then fill welding the 3 holes and filing round again. What do you think?