Whichever you shoot best is well said. And if you feel caliber makes a big difference then go that way. I used to think only a .357 mag with wild hand loads would be best, but now I protect me and mine with a .40. I have faster reloads and more shots per load than my .357.

Undoubtedly a magnum rifle will put out the most penetration and power, a shotgun the most forgiving aim, and a machine gun the fastest follow up shot. I have shot all of them, and actually shoot them quite well. If the apocalypse happens tomorrow I'll gather a couple machine guns and grab as many 9mm pistols as I can find because 9mm ammo will be the most common on a battlefield. I'd also get a rifle that shoots full metal jacketed rounds capable of penetrating vehicles.

But lets get back to what you need a weapon for now, pre-apocalypse.

You want something that you can shoot well and often, and you can teach others to shoot. If the weapon isn't second nature to you, if you can't completely operate it in the dark, if you have any hesitation, then you have the wrong weapon.

BTW, I just ordered a BG 380 a bit ago, and will carry that. Sure the power isn't the most, but it's nice and small and I never expect to need to pull it out. It's a tool I need to carry everywhere but probably never use. Comfort first in that case. If I ever do need to use it, God forbid, well then I will be proficient enough to place 6 good shots and increase my chances of going home.