I am currently working on a subsonic load for a 300 blackout. AR platform, 8" Wilson Combat barrel, standard gas port size and gas block, obviously pistol lenght gas system, 1-7" twist. I am attempting to work up loads using 150gr. Hornady fmj-bt, since they are relatively inexpensive. These will be plinking rounds only. I have been using IMR-4227, which shot fairly well, functioned well (with Silencerco Hybrid 46), but I was getting a good amount of unburned powder in the receiver/bolt. I have Lil' Gun and Accurate 1680 on hand, and will start working from the minimum advertised charge and go down in .5 gr increments. If anyone has a similar load/experience with this setup or end goal, please give me some direction. Thanks!