I recently competed (poorly, I must add) in the Berger SW LR Nationals in Phoenix AZ. But my success (or lack thereof) is only secondary, and has very little relationship to the efficacy of using the .223 for longer distances. My failure was of medical origin, not mechanical. More importantly, my Granddaughter competed alongside me, and the success of the expedition concerned her, not me. Her first organized Highpower (F T/R) match; coming out the other side with a smile was achieved, and highly satisfactory.
We employed a pair of .223 rifles, both with 5.56 chambers and 24" barrel lengths. We also employed the same handload, a 75gr HDY HPBT-Match bullet in unprepared Win brass with a close but not Max charge of Varget, and CC BR-4 primers. This bullet because my rifle, a Savage 11VT .223, has a 1:9" rifling twist; while my Granddaughter's rifle was a Stag Model 6 Super Varminter, with a 1:8" rifling twist. This load was tested in both rifles at 300 meters, and performed well within reasonable expectation. The rifles will not process (feed) or shoot the 75gr A-Max reliably due to its greater length.
Once the handload was verified in trainup, it was set aside, and all marksmanship training was performed using factory Prvi-Partizan 55FMJ ammunition at 100yd. For training purposes, at 100yd the inherent accuracy of the load in these pretty accurate rifles was highly adequate to the task. At significantly longer distances, it would probably not be truly indicative of the human segment of marksmanship performance, but we weren't training at longer distances, so 55FMJ was fine.
The match was fired at 600yd, and every shot my Granddaughter fired for score was a 9 or better. I think that's a highly adequate performance for a never before shooting beyond 250yd beginning competitor.
Undoubtedly, the rifles, chambering, and probably the loads, are sufficient for longer distances. But pushing the envelope was not on our agenda.
Greg