The 44Mag loads would have been too much nose and blast for the speakers on the movie set probably.
I remember reading that in the first movie, they actually used a 41Mag as S&W weren't producing 44Mags at that time and the movie needed to go ahead, so a 41Mag it was (PS, Don't tell the 41M guys, they are like Vegans and we will never hear the end of it, haha.