Keith has it exactly right. The Shockeys Gold is giving you much less velocity than the Blackhorn. The bullet's dwell time in the bore is longer and the muzzle rises higher before the bullet exits. This is the same reason a heavy bullet will usually shoot higher than a lighter one with the same charge. The heavy one has a lower velicity and longer dwell time.
JSG is a very low pressure powder. Just to give you an idea of how much of a velocity difference you are getting, I got a 5-shot average of 1048 fps in my Lyman Mustang with 100 grains JSG under a 300 grain Speer Gold Dot in Harvester crush rib sabots. With the same bullet/sabot and 100 grains Triple 7, I got a 5-shot average of 1822 fps.