proably the most used powder by guys that actually get an ar to run with 300 bo subsonic is 1680. Id try that and a bullet at least a 180 grain, preferably over 200. Might have to change your recoil spring if its an ar15. Most 762x39 ar15s run a 308 weight spring. Maybe a standard carbine spring and a standard 556 weight buffer. With the longer case a 762x39 has over a 300 bo you might run into problems with seating some bullets. You will be on the ogive (tapered part of the nose) when you seat a bullet that heavy in a 762x39 case and keep the overall length short enough to fit in an ar magazine. Its kind of why most who want sub sonic don't even bother with the 762. They just get a blackout.
MAKE AMERICA GREAT AGAIN! STEEL FOR TANKS NOT FENCES!!!