Hey Raff.

What is your twistratio?.

I could makeup some 900 grain conicals for you in .775cal. You shall know that using smokeless powder like BlueDot create less chamberpressure than blackpowder. If you are using strong brass like from RCM, I wouldn`t worry at all using BlueDot. The funny thing about the blackpowderloads is when someone wants to try to emty the "magazine" of your doublerifle..Recoil hahahaha.
Please post a picture of your double when time permits so

When I shot my double for the first time I was using BP loads, cleaning was with regular dishwash sulpher, and the barrels were mirrorbright afterwards. Smokeless loads are just easier, but I do enjoy the blackpowder too. It is simply a joy to unleash full charge BP + 900 grain lead. One really gets understand what power these borerifles are producing.