Greetings @Sakoswarosorted,
After many years of loading the .223 I have settled on 25.5 grains of AR2206H behind the 55 grain Hornady in US cases. This produces 3,130 fps in my T3X and excellent accuracy in all the rifles I have tried it in. Your Lapua cases are a lot heavier than the cases I have been using and capacity may become an issue so for them perhaps 25 grains would be the Magic load for projectiles in the 50 to 55 grain range suited to you 1 in 12" twist.
Regards Grandpamac.
Bookmarks