the issue I have had with 300 black out is it was designed for subsonic use the some loaded it up to supersonic but it's not ideal at those speeds. 7.62x40 is the same case a 5.56x40 necked up and trimmed to 40mm but was designed as a supersonic round so it can be loaded hotter and hit harder. but they both perform the same or lower than a 7.62x39 both in FPS and FTLBS so the ability to find and shoot relatively cheap ammo puts the x39 ahead for me.