Just had a Interarms MK X in .223 handed to me, it a 20" barrel 1:12 twist and I have loaded up some 40 grain sierra BlitzKing and some 52 grain Sierra HPBT using 4 different powders and am going to try and get out this weekend to evaluate them. Maybe have some load info next week. I'll post them here when I get it figured out. They guy that gave me the rifle said it wouldn't shoot factory 55's.