there is no advantage to a 300 blk out or whisper compared to the 308 for a subsonic bolt riffle set up. The main purpose of those rounds was for a heavy subsonic bullet in the AR platform. I think there is an advantage over 308 in AR platform as all the AR-10's seem to be significantly heavier. If you are running a bolt rifle I don't know or see why you wouldn't choose the 308 so when you are shooting supersonic you have more performance. I haven't ever understood the bolt guns chambered in 300 blk out.