> Thompson Contender, which he claims can easily change calibers just by
> switching the barrel. I don't know if this is true

Definitely true; I have one myself. Not a self-defense gun, though, despite what you might see in some John Woo movie, as it's a single-shot. Good for target shooting and hunting (in appropriate calibers). Great fun. You can set it up as a nice light rifle or big pistol. For larger calibers they have a Thompson Encore now, I believe.