I know a guy that casts beautiful bullets for a lot of guns. 45/70, .30, 8mm, etc. He's a credit to the hobby.
Today I heard him say with regards to shooting cast out of an M1 Carbine, "You can't do that, you'll lead it up, the twist is too fast and the rifling too shallow for cast lead."
Didn't matter that I shot a better group with cast than with 110 round nose jacketed, or that I told him I had about 1000 or so through said gun with no leading and no function problems, running about 1900 fps, he was adamant that I was nuts.
Go figure.