This is pure speculation, but one might assume when firing a 12ga wad through a 10ga bore, there would be an increased chance of the shot making actual contact with the bore, through the wad opening up, spreading the slots. It is necessary to prevent bore contact with these harder shot to prevent scoring. Choke bulging is of course another situation altogether. You would likely have no problem there with use of the proper choke tubes. I do not think I would risk shooting 12ga wads with hard shot.
