The gun was originally made for black powder, so I would be inclined to start with black powder loads, more or less fill the case up with 2F or 1F grade powder. Be sure to use a drop tube while loading.
I think that if you are using hollow base bullets (mini balls) , the thickness of the skirt is going to be extremely important to achieving accuracy as well as using a very soft alloy of lead. You may also want to try paper patched solid base bullets.
The greenhill formula (150 diameter square over twist = length) will tell you the maximum length of slug the rifling will stabilize.
Finally, my experience with black powder in general has been that seating the bullet just short of the rifling is more important that the actual charge of powder, in achieving accuracy
cheers Doug
