Xavier has written about a guy to got his thumb blown off by a .460 revolver.
It is important, when firing a revolver, to keep all parts of your hand(s) away from the front of the cylinder. Explosive ignition gases exit the cylinder gap as well as the muzzle of the firearm when the cartridge is fired. This safety issue is sometimes illustrated by instructors who place a revolver inside a paper bag with the muzzle sticking out a hole in the bottom.
Xavier has also posted the photo of the thumb itself. It is not pretty.