M855 and M193 is ran at higher pressures than your 223 remington.

I shoot alot of M855, and can hit "area" targets with it at 500meters.

Man's torso is hard to hit but very doable. 55 and 62gr are really designed for 300 meter human targets and 400 meter area targets.

But like some said, if you load 75gr high BC, 500 meter human torso targets are extremely doable.