Look at 572 powder in a straight walled hull like Fiocchi which will be 7,700 psi from Hodgdon Data. I find the hulls on GunBroker for about five cents each or less. Do recall that smallbore pressures are higher than large bores. They are proofed at 100 bars higher pressure so their service pressures are higher than the 12's you are use to shooting. Also make sure your new gun is proofed for nitro and check the barrel wall thickness.