[M Kehoe]

For human targets at 500 meters or less the 5.56mm (.223 cal) and .357, 158 gr. CJH (with a 'cross' cut on the bullet) at close quarters.

The NATO 5.56mm tends to 'run' and create havoc. The .357 158 gr. CJH tends to knock a perp down no matter where you hit him (and probably won't go through a wall afterwards).

5.56mm

[medved]

308 Is your most generally useful rifle calibre. It is inherently accurate and efficient, it can kill just about anything, certainly anything which walks in North America, you can still buy surplus fmj 308 ammo very cheap and that's likely to stay that way even with military rifles now being 223 because NATO machineguns are mainly 308.

If you work hard enough at it like building a very light, short-barreled bolt action, you can get serious recoil from a 308, but 308 recoil in semiauto rifles is generally negligable.

If I could only have one rifle, it would probably be the Kalashnikov Vepr II in 308. Those run about $500.