Nothing says big freaking hole like a 12ga sabot, LOL.
The 12ga shotgun is defiantly the most versatile survival weapon, its capable of taking any size game by simply changing ammo. Number 6-8's for squirrels and rabbits and 00buck or slugs for large game and Zombies. The downside is the ammo is heavy and you cant carry a lot of it. What I would take or carry would depend greatly on the scenario and rather or not I was the only survivor or if I had family members along.
My guess is that more than likely I will take whatever I can get. Usually when goes wrong it all goes wrong. If in your grandiose plans you mean to have an AR-15 and a side arm and a bunch of ammo, all decked out in all your tactical gear. Chances are you'll end up with a Jennings .25cal Saturday night special and 20 rounds in your P.J.'s. I mean what do ya do if there has been some sort of detonation and your gun safe has toppled over and cant be moved or its damaged and bent and cant be opened, then your stuck with your nightstand gun and whatever ammo wasn't in the safe. WOW that's a bummer I wish I hadn't even thought of it..
I agree that its fun to chat and imagine going out into the wilds of a defunct civilization and being like the ultimate warrior but in reality chances are we are just gonna be clawing threw the mud for anything we can get...
Wanna real bummer to consider? One very real SHTF possibility here is that of a tornado coming through and wiping out a town. Then we'll need our guns to deter looting, etc. Except (especially for those of us without basements) our guns and ammo are liable to be in the next county!!! Not sure what to do about this possibility.
So much depends on the setting, an AR is a good choice, but in our flat lands, I feel a heavy caliber, something in 30 is needed. I have gone with .06, but there are other good choices.