"Zombie Apocalypse" SHTF situation it would be a .22, probably the MP5 clone because it holds 25 rounds.

Actual SHTF (what I voted for) if I had to run to the mountains and only take 1 gun it would be a shotgun, 870 express 12 ga. I have slugs, birdshot and buckshot on hand. I can hunt nearly any game available and protect myself.