[RyeDaddy]

Well if you think about it a 300 BLK is a 5.56 case with the neck blown out wider, so it really couldn't hold more powder. 300 BLK tends to be used with magnum pistol powders like H110 for the supersonic loads, so they burn up sooner in the barrel than common 223 powders like H335 and Varget. Also compounding the issue is the larger bore diameter, the gas has more space to fill up to develop the same pressure. So by the time you reach the gas port on a rifle length system the powder is all burned up and there isn't as much gas volume or pressure to operate the action. I wouldn't advise a barrel longer than 16" for sure, and I would stick with carbine gas for that barrel, or pistol length gas for anything shorter. Hope this helps.