This is a pretty open-ended question, but I'll give it a shot.
Disregarding legal questions other than Federal laws (which I think is what you intended), I'd recommend either a .45ACP or a 9mm semi-auto. Both of these will get the job done, which one you pick is personal preference. But both have several generic advantages, as follows (not in any particular order):
1. Easy availability of ammo
2. Presuming a decent weapon, parts/repair is straightforward, as both are as common as dirt, usually.
3. Without getting into a tribal thing, both have adequate lethality.
4. Both are capable of any level of accuracy that a given amount of practice can render.
5. Both are available in a wide range of weapons, to suit almost any preference regarding brand, or operating characteristic.
Lacking the ability to carry a gun, or a knife longer than 3", I'd learn stick-fighting, boxing, and running, plus acquire some conflict-avoidance training.