When To Choose A .308 Over 5.56?

[markm]

How effective is the 223 M855A1 on a no body armor target at 600 meters vs the 7.62 x 51 M118LR?

I'd love to get my hands on some of the projectiles out of M855a1. There've been posts here from guys who've seen its effects on torsos and heads and stuff. Supposed to be pretty good at tissue damage.