This myth was a bit of a shocker.
Taken from the same episode as the SKS firing pin malfunction, the myth claimed that a person who grasped a Smith and Wesson .500 revolver improperly lost his thumb when the exhaust gas from the gap between the cylinder and barrel blew it off.
Seems this one did was more than valid. Assembling two mock hands made of chicken skin, meat and bones, then wrapping the 'hands' placed in a similar cradle hold around the cylinder, the test was conducted two times.
The first test produced significant damage to the 'finger' closest to the gap. The second test ripped two of the 'fingers' apart, smashing the bones. The high speed camera gave a very graphic image of the sequence.
Lesson learned: Never cradle a large caliber revolver by holding your fingers anywhere near the front of the cylinder!