I prefer 308/30-06. Have a AR-15 with lots of toys hung off it, but my serious guns are both bolt -30s That said:
In my experience, the mil-surplus 7.62x39 is worthless crap. If you're buying it to hunt with, and want to hit reliably outside 100 yards, you have to buy expensive ammo anyway. Also, the surplus bullets are non-expanding.(actually, the 7.62x39 ammo we get now isn't true mil surplus anymore, no loss there,...) again, no good for hunting.( EDIT: I know, you said no hunting, but the accuracy problem is there even if you just "hunt" paper)
Having said that, I'm partial to the ranch rifle in 7.62.(I reload, so ammo cost isn't a factor) for the bigger bullet. If I was stuck buying factory ammo, I'd get the .223.
On the .223: X2 what 8mud said. Get a gun with the faster twist rate. A lot of newer factory loadings for the 223 and all the Mil-spec is 62-65gr, and most of the older 1in12/1in10 twist barrels won't stabilize the bullet.
Choice of gun, if you want to bling it out with light/dot scope/HUD/laser, etc, get an AR. Lot more toys, less weight, and a whole lot easier to put together.
For an actual "all hell broke loose" gun: Short AR-10/SR-25 or the Springfield SOCOM-16 or Scout squad(not the SOCOM-II, that big aluminum POS on the nose is useless ballast)
http://www.springfield-armory.com/armory.php?model=19
'Nother nice one, the SA-58 series:
http://www.dsarms.com/SA58-FAL-Carbine-Rifle-308-Cal/productinfo/SA58C18/
You might argue about price of the gun, but then, if all hell did break loose, do you want a cheap gun?
My actual "all hell broke loose" gun :
http://www.steyr-mannlicher.com/index.php?id=14&L=1