I ran some numbers on a ballistics program, and it does not appear that there is as big a difference at long range between these two rounds as I thought. Are my results correct? I kind of hope so, because I would much rather put together a SPR type AR-15 than switch over to a 700, M1A, AR-10, or FAL for longer ranges.
I compared two Sierra BTHPs, 69 grain and 168 grain, as loaded by Federal. While you can put heavier bullets in both cartridges, I thought this would be a good comparison because these seem to be popular match bullet weights. At 1,000 yards, with a 200 yard zero, the 223 drops 415” and the 308 drops 402”. So there is no significant difference as far as bullet drop.
In regards to wind resistance, with a 10mph crosswind the 223 is moved 41” at 600 yards and 140” at 1,000 yards, and the 308 is moved 34” and 112”. So while there is a difference here, 21% and 26% respectively, it is not earth shattering. You might need to be a little better judge of the wind with the 223, but your skill in this regard will need to be well developed with either cartridge.
The 308 has a clear advantage as far as energy, but at 1,000 yards the 223 still has more energy than a 22LR at the muzzle. So while I doubt you will knock a guy off his feet at long range with the 223, he is probably not going to keep coming either.
And yes, I am ignoring cover. But I figure you need to see what you can hit.
Am I up in the night here?