FYI concerning shooting a bullet larger than the bore and the bullet swaging down, a friend of a member here accidentally loaded(don't ask!) and shot a 7.62x39 round in a .243 Handi a couple years ago, the bullet exited the bore, but the pressure damaged the underlug and they were unable to open the action, the rifle was returned to H&R for repair, due to underlug damage, they replaced the barrel, the frame was undamaged.
Tim