Me personally will pass on the ".300BO" flavor of the month. After chatting with a few of my AR buddies who did the conversion both commented "It's all fun and exciting when ya got a new upper and caliber to shoot, but then it simply gets old and boring". Shooting .300BO subsonic has some coolness to it, but that's about it. From a recreational weekend shooter viewpoint it just doesn't make any practical sense to stuff expensive and heavy .30 cal bullets into a modded .223 case. But I also all for "do whatever you enjoy..." 
