I'm not sure if this qualifies as a myth about guns to scare people, but a lot of people don't seem to understand how different calibers work. When I say ".50 caliber pistol" most people seem to think that I mean the .50 BMG in a pistol, because that is the ".50 cal." I have to then explain that there are .50 AE, .50 GI, and .500 S&W, and that while they all use a .50 bullet they are all different.