I need some education. I've searched the rimfire section but have not found a definitive answer. I'd like to put a 22/45 target upper on a Mk III std. I've tried to install but it doesn't want to go on even after rapping it harder with the mallet than I'd like. If they are not interchangeable I'll stop of course, but if they are I may try to take a little material off the roof of the 22/45 receiver's recess that accepts the frame's square lug. Anyone with words of wisdom?