Seriously, half the world is currently in a near starvation situation and they aren't eating each other; lot of post-apocalyptic writers like cannibalism because it sells books, but the facts aren't there to support it. Lots of evil will be going on for sure, though. But we've been talking about a grab and go, staying low, not being seen, and being on the move for 6-9 mos (1st winter? - ngh).
If you haven't done something like this, I highly recommend it. Get the pack you would want to carry for as long as you think you would need to, and put everything in it you think you absolutely need BESIDES guns an ammo. Put it on - most people have now discovered that they packed too much. Once you've got it down to where you think its okay, go camping for a week, and walk at least 10 miles every day. When you get back, you will most likely make changes. Repeat until comfortable. Weigh your pack - mine comes in near 40, but there's alot of beans in there. I'm a 40 yo 6'4 220# military guy who enjoys endurance tests like distance running, hiking, so if its not light, rugged and versatile, it doesn't go in the pack.
NOW take all the guns and ammo you think you'll need to stay alive and eat (1 shell per rat, 1 rat per day?) and weigh those. Because of the 12ga pump affinity on this thread, I took my 37 (synthetic stock, 18.5") with a full tube and weighed it - 8lbs. Add a full side saddle and a scope, and I'm guessing 9+ lbs. 10 shells mixed slugs and shot weigh roughly 1lb on my scale, 50 shells = 5lbs in the pack. Extra barrels have to go in the pack at 2lbs each. I've not used my pump enough to be able to estimate any extra gear or tools I'd need for 6-9 mos, but throw those in too. So at minimum, two barrels & 50 rds, you're talking 15-16lbs for one gun option. Take it camping for a week - don't have to shoot it, just be sure you're carrying it with you 24-7 - never more than arms reach away. Again, 10 miles a day but with the weapon at the ready, not slung, to simulate how you'd carry it in a crisis. I'm tired thinking about it, and I've humped a 60 (machine gun) for many miles and many days.
Ballistically the 12ga is a superior round to a .22; no argument here. It can drop anything at 100 yds - got it. Its a proven man-stopper - yep. That's why I have several. But my last and only gun to carry with me through whatever may befall, with all the ammo I might ever need? I have to think beyond ballistics, and think tactically ... efficiently. Is the extra carry weight worth it? Not for me.