The reason that happens is two fold... T7 is engineered to match the black powder performance curve, so it burns very rapidly. In otherwards it reaches it's pressure spike very rapidly... on the other hand BH is engineered to meter and perform like a BP but it is really a smokeless progressive buring powder. It burns slower so it never reaches the high pressure spike that BP or T7 does, although if a heavy enough bullet is used it will created more pounds of pressure total than does and equal amount of T7. But because the 220 grain bullet is so light it gets out of the barrel before BH can realize it's potential... add another foot to the barrel and BH would create more velocity.