In that kind of situation, I have to go with one of my Tokarev, TT-33 pistols. Easy Concealment would be paramount. HARD hitting power is next. Enough power to stop a large in shape person.
I think this is why infantry carries something like a TT-33, P38, CZ or, 1911. These can drop your enemy when your primary weapon fails. Semi Auto pistol.
You can carry a lot of ammo.
Easy to conceal.