I have several long arms and hand guns. I suggest my favorite...
Ruger .357 magnum...stainless steel...rubberized grips
Reliable, easy to use and clean, can also fire .38 rounds, has a huge psychological impact on your opponent.
A smaller calibre will just piss off your opponent. The semi-auto handguns are probably better, but you have to deal with magazines, safeties, and the
first round has to be chambered.
Maybe I'm a traditionalist/ old fashioned...but nothing is more intimidating than that big ol' barrel pointed at you and you can see the hollow