I'm working up a 150-gn 6.8 subsonic load and the first thing I can tell you is, you just won't get there with a 1:11 barrel. I'm ALMOST there with a 1:10 barrel, but it's a struggle. I recommend you start with a relatively-short 130-gn bullet (nothing with a plastic tip!) and try either Trail Boss or IMR SR-4759. Also, you might as well close your gas block 'cause unless you're prepared to make some real hardware changes (super-light bolt carrier and buffer, reduced force buffer spring, pistol-length gas system opened wide) you won't get it to cycle with subsonics.