Wouldn’t it have to be something like a .45-90 or a .50-90?
Actually, I think I’d start with a .45-100 or 110, put 90 grains in it, mark it and cut it down to the perfect size.
This site list cases in grains of water which i have read is close to black powder in grains. Pyrodex IIRC is equivalent to black powder when measured by volume.