A lot of this is very subjective and includes the environs one intends to do their surviving in, what one can shoot well, etc. No single weapon will be the ideal for both harvesting squirrel and warding off grizzly bear attacks. Concealability of a weapon may be critical in some circumstances and not an issue in others, etc. Of course one who planned ahead would start out with a good stock of ammo already laid away, but sooner or later it will exhaust itself and then either availability, foragability or reloadability of ammo becomes a consideration. While I have a few firearms that suit certain needs better than others, if I have to grab but one and make a run for it, I’m slinking away with my folding stock M1 carbine.
Like you my first grab is my M1A. .308 rounds will take just about anything I am going to encounter in North America. Squirrels, sorry but I have not problem blowing the little punks and going for the head if all I have ia my M1A. Then again if there is a deer, coyote, mountain lion, bear or even a seal .308 185gr is about all it is going to take.