Had a guy doing something similar at a range I volunteer at.
Couldn't find .45Colt to go plinking, so he just bought .44 magnum instead. Hey, it's smaller, right? And it fits...
Hits were all over the place downrange, and recoil was beating the guy up something fierce. RSO goes over 'cuz he notices something is just "wrong," and sees the situation. Removes the guy from the line with instructions to get his gun checked out by a smith and never shoot wrong ammo in his gun again.
It blew my mind when I heard this story.