H322 gives me the best accuracy with 77 grain SMKs out of all the powders I have tried. Only problem, is that load is slow at 2550 fps.
scope adjustments can take care of that...what distances are you wanting to shoot with it...
a slower 77 grain SMK, is still flatter shooting than a faster lighter bullet in a 223...
or the other option... a fast twist 22.250... I've got one with a one in 7 twist.... she will reach out there...