I have two Taurus RBs in .454 - one scoped with a Burris 2x-7x and the other unscoped.
With the scoped version, I too can get great groups, but not as great as yours. Specifically, I get about 3/4 inch groups or less at 25 yards. Not bad for such a shorter barrel. With the unscoped version, I get about 1 inch at 25 yards.
It seems that it I get the same groups regardless of what factory ammo I use.
Zachary