I would put the weight as far away from the barrels as possible. That way a smaller weight, near the butt plate will be able to balance the heavy barrels. Theoretically you could do the same thing with a greater weight nearer the hinge pin but why make a gun any heavier than needed. You want balance not extra mass to tame recoil. But even a balanced gun may not swing well. Heavy barrels can be balanced with weight in the butt but they are still heavy barrels.