My opinion for SHTF guns are to plan for the ammo not the guns themselves. In a post apocalyptic zombie horde scenario think of where you can obtain
ammo. Unless you plan on lugging around ammo cans or cases of ammo for your guns you may have to scrounge or barter for more.
Military and law enforcement typically use 5.56mm (.223 Remington) as a rifle round and 9x19mm Parabellum (9mm Luger) for pistol rounds. 12 gauge
shotgun shells are used by police, military and millions of sportsmen.
.45 ACP and .40 S&W are also pretty popular semiautomatic pistol cartridges and have a bit more knockdown power than the 9mm.
Personally, I'm partial to the .308 Winchester (7.62x51mm NATO) for a reach out and touch someone rifle round. It's popular with sportsmen and
military and police snipers as well.
Unless Russia or China invades, chances of getting your hands on some AK or SKS ammo (7.62x39mm) after you run out might be tough.
Just my two cents.
edit on 27-4-2011 by survivalstation because: forgot to include the .308