I'd suggest you look at
www.long-range.com for some suggestions by the experts at 600 yard shooting of the .223.
I'd look at H-322 and BL-C(2) for propellants. I use 'BL-C' myself, because it meters so well....but H-322 will provide excellent accuracy if that's what your rifle likes and if you load your cartridges properly.
I do not shoot in any matches, but prefer to load for varminting accuracy and for the use of .223's with Hornady 55 and 60 gr' spire points on White Tails by a couple of land owners I know.
Take care.