If u already have this barrel assembly in 5.56mm & attached to an upper with ano M16/AR15 BCG, all u need to do is get another one of the barrels assemblies chambered in .300blackout, swap em out & ur ready to go. U don't even need to buy any new mags. .300blackout was designed to fit 5.56/M16/AR15 mags & used with AR15/M16 BCG's. For any AR to be able to fire .300AAC, all that's required is for a barrel chambered for .300 to be installed. That's what makes this system from DiamondHead so cool IMO. & with the possibility of extremely ridiculous gun restrictions coming in the near future, a system like this would be great to have. Not just for the ability to swap calibers so quickly, but also being able to quickly remove the barrel essentially making the rifle a non functioning firearm. Or if u live in a state where u can't have an AR pistol, u can swap from a pistol to 16" rifle very quickly. Lots of advantages to to having this system.