ok, 100% works with me now, just pick shotgun, and whilst holding fire press switch, you should have the amount of bullets from the other weapon (srry i didn't wit to put it all in one post)
lol glitch: just mess around with shotgun and mp50 or ump, use your submachine gun and play with the "shoot", "reload" and "switch weapons", if lucky you get your shotgun with 25 bullets :P