Glock in .45acp or .40cal. Its concealable, the ammo is easy to find, it will shoot forever, and its stopping power is almost flawless.

I would go with a rifle other than that, but walking along solo with an AR/AK type platform visible just paints a target on yourself and makes other needy people want to jam you, jack you, or worse.

If I had say 4+ guys... thats a whole different story, but ill just say that a full tac kit / bugout vehicle / mobile fire team would be useful.