With that 1/12, you are going to be limited on bullet weight in .223. You might try Sierra's 60 HP for 300 meter shoting, it MIGHT stabilize in your 1/12 barrel. You probably are going to be limited to the 52 and 53 match bullets, try Sierra Match Kings. Your barrel will not stabilize the 69 Sierra Match King.
1/9 twist barrels are generally good for up to 75 or 77 gr .223 bullets.(Sierra, Hornady, JLK, Berger)
1/8 twists generally will handle up to 80 gr. .223.(Sierra, Hornady, JLK, Berger).
1/6.5 twist is required for 90 gr. .223.(JLK).
The determining factor in whether a given twist will stabilize a bullet is the length, not the weight. Of course, generally, the heavier a .223 bullet is, the longer it is.