What down range power?? Any round ball has such a poor ballistics coefficient that it loses velocity fast, and with it, power. The Faster you send it out of the muzzle into air, the faster is slows down. If you study this issue, you will find that any increase in down range " power " relates to an increased WEIGHT of the ball or bullet, not MV. You are not going to appreciably FLATTEN mid-range trajectory by adding more powder. Maybe if you are shooting a high power SCOPE sight on the gun you could SEE the difference in how you hold the crosshairs on target, but with iron sights, you are kidding yourself.
No one can tell you the Most any-kind-of powder you can shoot in that gun or any gun. The age and condition of the gun, who made it( really- not who sold it) and other factors all affect how much pressure the barrel will take before it comes apart.
YOu are shooting a very large caliber, heavy RB in that gun, and you simply do not need to up the powder charge beyond the figures already mentioned. Your shoulder will tell you that even the high end charges mentioned will be a bit much, considering the shape of that stock, and its steel butt plate. Eurpean Made replicas may Not have the same quality barrel steel in them that you find being used here in the USA by private barrel makers. Keep that in mind, and enjoy shooting the gun with what you have. :thumbsup: