Survival is not Special Forces
I own 2 12's (an 870 pump, and an O/U). I also own 2 Savage 24's, which are combo guns with .22lr on top and 20 gauge on the bottom.
First off, a 20 gauge can do ANYTHING a 12 can...just at shorter distances. Period. YES, you will obviously have less margin for error with the 20ga. But assuming you do as much shooting as you do talking about your guns on internet forums...you're just fine with a 20 ga for survival purposes. In fact, I'd say you're even better off.
The reason? Well, as I said, any of the bigger purposes (e.g. slugs for deer or black bear) you can do, just not with as great a range. Again, can you shoot? Secondly, for smaller game (e.g. squirrels, rabbits, and upland fliers) the 20 is the right amount of medicine, where the 12 is more likely to make 'pink mist.'
The argument some make about the military and police departments using 12 ga is poor, because you have to consider THEIR purpose. First off, they need to shoot at human size targets. second, they need to limit margin of error. So size, weight and recoil constraints are of no concern. Nor is overkill, which most CERTAINLY is if you are looking at survival and want to "hunt" meat, not just "kill" meat. You want to have enough of it left to eat.
I've said on a rifle thread, and it bears repeating on a shotgun thread, that survival is first and foremost about getting food and shelter. Not building an arsonal of "tactical" weapons to defend a foxhole against hoards of invaders.
If you're smart, when faced with a survival scenario, your first move is to
get away. You need to get to where you can be low-key, hunting food. That means small ground creatures, by and large, as they're everywhere. I don't want my 12 ga 870 pump when I need to kill a rabbit. I WANT that little Savage 24 and the .22lr or 20 ga, depending on distance.
I would rather be fending off bad guys or ornery bears with a food gun...than trying to hunt food with a weapon more suited to war.
Survival is not a Rambo fantasy with plastic spray can guns and a very handy, unlocked, well-stocked ammo depot nearby to raid. It's cold, you have little supplies, you might be hurt, and you need high odds food options. 20 feet into the woods is a squirrel or rabbit. A woman or young adult can shoot that little 24, if necessary.
So...which is better, 12 or 20? Depends on what you already have, and what you need it for. Gun to the head, I would likely pick up the 20ga.
Oh, one more thing. The 12 is the #1 produced caliber, the 20 is #2. So while yes, there will be more 12 ammo about, it isn't like you wont find 20.
Better bet...stock up on either or both. But do so in mind of what you will encounter to eat. NOT based on what you think makes you more like "Ahnold" in Predator.:thumb: