Ulfhere-

Have you owned/shot a double rifle before?
Developing a load for your double rifle will demand a different process than you're likely accustomed to from shooting your single barreled rifles.

The first thing you *need to do* is cast your chambers with Cerrosafe alloy or a similar material, so that you can positively identify your chambers one way or the other.
You also need to slug your bores to make double damn sure what bullet size to run.

If you're going to have someone make you dies for your rifle, you might as well be prepared to send them a chamber casting anyway so that you have *exactly what you need*, granted with the low operating pressures of Black Powder Express cartridges you'll likely only need to neck size as the proper load won't be pushing your brass very hard at all.

Now I'm going to paste some words from another member, from a response he'd given very recently to someone in a similar position. A few words have been changed to suit your situation...

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"You said your rifle is a double. I assume you're new to them. It isn't a single barrel rifle, so don't try to treat it like one. Everything changes.

Double rifles are wonderful general purpose hunting rifles. Much of their beauty is in their simplicity. They're regulated for one standard (full power) load. In handloading for one, once you've duplicated the load it was regulated with, you're set. All you have to worry about is that one load, and one bullet in that one weight. Don't start out trying to make light loads or "pet" loads of other shooters work. Barring luck, they won't regulate and will only frustrate you. Yes, reduced loads can be made to work in some double rifles, but that's for later. To start, you need to find the load it was regulated for.

Since most rifles like yours were regulated with a standard shape and grain weight bullet, but not all, I'd start with that weight. They were also built to shoot with a standard load of powder at a relatively standard velocity.
For the sake of standards, there were no victorian-era double rifles built to run sharps cartridges. The sharps cartridges ran much differently than express cartridges, typically with much heavier bullets at lower velocities, the express cartridges ran lighter bullets with flatter initial trajectories.
You'll need to prepare yourself with some knowledge of the original black powder load, then use a standard nitro-for-black conversion ratio, then start a few grains low, and work up shooting over a chronograph. Don't fire multiple rights and then multiple lefts. Fire in pairs in right/left order (that's the order they're regulated with). Note where the rights strike relative to the lefts. Generally, if they consistently strike wide (right on right and left on left, several inches apart), velocity is too low. If they cross (right on left and left on right), the velocity is too high. If the barrels are still wide at suggested proper regulation velocity, STOP, and try another powder. Perfect regulation puts the rights on the right and the lefts on the left ~3/4" apart at 50 yards.

Like I said, everything changes. When handloading for Black Powder Express double rifle cases in drop-down barrel actions, you can't use conventional pressure signs to keep you out of trouble. By the time they appear, you're over proof pressure. I never attempt load development in a double rifle without a chronograph.

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It's likely that getting your rifle so shoot well will take a little bit of research and take you on a bit of a
*'Nitro for Black' handloading for double rifle*
learning curve.
Take the "double rifles are different from other rifles" approach and distinguish the important and unique characteristics of the effort before you dig too deeply into loading/shooting bench time.
That'll save you time in the end, and possibly save you quite a bit of hassle and money in the mid-long run.