Truthfully, if I had a family member that wanted a 300blackout, I’d suggest staying away from subsonic ammunition unless it’s going to be a suppressor host. It costs more. It’s has a potential to be more difficult to get running reliable. It’s dirty. But.... If you run subsonic through a suppressor, it’ll make you giggle. Throwing a 30cal projectile down range and it being stupid quiet is absolutely fun. You will shoot a suppressed 300blackout much more than the round count your anticipating. Of course, that’s just my experience.
I do agree with piecing it together, but I don’t know your experience with the ar-15 platform.