if you're handloading.
any brass
Rem 7.5 / wolf SRM / wolf SR.223 / CCI 450 / any other *hard cup* primer
24.1gr of Reloder 15
77gr SMK
if you drop any 9's, it'll likely be your fault.
if you wanna go all out, my go-to 600yd load is:
Lapua Match .223 brass
25.0gr Reloder 15 (do not exceed 25.0gr)
80gr SMK, loaded at least .005" off the lands (they're VERY tolerant of jump, .005" is a start, i used .010", and load em 1 length all season *i dont chase the lands*) DONT JAM THEM INTO THE RIFLING!
Rem 7 1/2 SR benchrest primer ( wolf SRM / SR .223 give extremely load SD's as well )
that load is a HAMMER. 2750+FPS out of my 20" service rifle barrel, 2900FPS out of my 26" match upper.
*edit
do some reading on NationalMatch.us
there's really no "load development" that needs to be done for highpower. people have shot literally billions of rounds out of these guns, and the go-to's ALWAYS shoot well. the 10ring is 2MOA, so the "ideal" load would shoot 1MOA. you dont need a 1/4MOA rifle / handload to shoot good scores. your time is going to be better spent putting rounds downrange and working on building good, solid, repeatable positions.