The big challenge for 223rem getting to 1,000, besides the wind - which is a challenge for EVERYTHING at 1,000 - is the fact the tiny case doesn’t pack much horsepower. It’s pretty common to see 223rem loads falling transonic shorter than 1000. A 77SMK, for example, pushed to 2800 at the muzzle will fall transonic around 850, so things get pretty interesting by 1,000. A 73 ELD pushed to 2850 will make it to 900, another 100yrds doing a bit of the hula.
800 is a much more reliable range for 223rem, but getting to 1000 is really just a matter of allowing enough target size to accommodate the transonic destabilization. Group dispersion (MOA) might double from 800 to 1000.