I see no reason why they would tumble in the bore, there is a long section of "bore size" that will keep it lined up.
But I'm guessing that they'll begin to tumble after just a short distance in it's fired ballistic trajectory.
If larger caliber ball, swaged/sized, too fit the revovlver actually worked well for accuracy it would have been common knowledge and recomended now for well over a 100 years.
I'm sure someone would have had that very same idea at least once in history,
I mean if a barrel shape projectile actually worked, nobody would have had to create the flat base bullet and pistol caliber round ball molds would have never had to be made.
Just sayin, :idunno:
It will send a chunk of lead down range though.