the longest kill i made with my NEF 223 (heavy) was just a touch over 400yds, gave a woodchuck a 3rd eye.
As to the "223s dont shoot well past 300yds" i think, like someone mentioned, its the shooter, (for the most part). I wish i could have gotten my NEF with a tighter twist rate, say a 1:7 or even a 1:9. My AR is a 1:7 and that thing can really reach out there.