With pictures it is obvious what breakeyp is referring to. First picture, remove the barrel retaining screw. Second picture, inside the receiver view of the barrel retaining screw and the what it butts against on the barrel. And finally, the very fast left hand (turn clockwise to remove) three threads per inch of this MkII. If someone cut the original threads off (about 3/4"), possibly to re-thread to a more normal thread pitch, then re-threaded and re chambered the barrel to full sized service ammunition, it doesn't appear it would leave enough metal in the chamber area for safety. Tom