I have experimented alot with subsonic and cast bullets with a .300 Whisper benchgun, trued 700 action, Gaillard barrel, Jewel BR trigger, etc. This caliber was made for subsonic, however accuracy increased as I sped things up. I could never get the accuracy at 1050fps. that i could get at 1600fps, must be a stabilization thing. I tried every powder out there and best subsonic results were with AA#9(same as H-108), known for this use. So if you can find AA#9 you might want to try that and if you can't get it to shoot good, you may have run into the same stability problem!!