On a MKII the outer tube is riveted to two "Bulkheads", one at the rear of the bbl and one at the muzzle. Think freeze plugs with a hole in the middle. Been many a Yaron since I seen one so no idea how they formed them but IIRC they were normal swaged head rivets.
If using a solid bar as a receiver, what you want are called "drive screws", essentially a rivet with a series of grooves and ribs down the shank, twisting a bit. Commonly used to hold nameplates and such on metal items, you drill an undersized hole and drive them in, the ribs cutting into the metal and the twist keeping them from backing out.
Drive Screws | Fastenal