Ben, I frankly don't know the ballistics. I never really worked out a load for my Horsley. At the time I was casually doing so, I happened upon my Purdey .500 BPE. My attention shifted to the Purdey, and loading for it. I never got back to the Horsley, and ultimately sold it about four years ago.

The Horsley had 2.5" chambers. The barrels were 28", Henry rifled with a twist of 54". The rifle weighed precisely 11 lbs. It was designed for paper cases, with a bore diameter (lands) of .729" - true 12b, and a groove diameter of .735". Lyman offers a round ball mould of .735". That is what I used for my limited work.

Note that the chamber dimensions are the same, whether the rifle is made to use paper shells or brass shells. The difference is in the bore dimensions. A paper shell loaded with a projectile of .750" diameter will not chamber in a 12b chamber. If you load the same projectile in the 12b brass case it will go in the chamber, because of the thinner case walls. You must determine the bore dimensions and match your measurements closely. This in turn, determines whether you use brass or paper.

This is not an endeavor you enter lightly. There is a lot to learn about these bore rifles. If you aren't an advanced ammunition loader, I would suggest you steer clear. I don't mean that as an insult.

As I said before, the 2.5" paper 12b case will just barely hold 6 drams of BP with proper wadding and a round ball seated with a roll crimp. That's the limit of the paper case. The brass case has more capacity. I don't know how much more, as I haven't use brass cases.

A roll crimp was the traditional closure for paper cases, and it results in more case capacity than a star crimp. It uses less of the case wall to effect closure.

Curl