I'd like to know how the brass chambers in the rifle, it may not need the sizing that is being attempted, and possibly the die is not sizing the correct areas. A reloader may have other dies that can reduce the shoulder of the case, just enough to chamber for fire forming.
I am reluctant to mention it, but I have not run into much difficulty reducing the diameter of a case head by squeezing it in a vise. I just drill a hole in some scrap steel then cut through it with the band saw. The hole can be over sized and the opening adjusted by grinding a little off the mating faces. I have cobbled up support so it sits between the jaws, then give brass little bumps where the die couldn't reduce it, turning the brass constantly. I sat a primer pocket uniformer in the opening, the neck is facing down and the rim keeps the brass from falling through, just in case it distorts and do a clean up cut on the pocket.
As mentioned earlier, it may be frustrating to keep trying with the sizing die. The seating die can be an option. If it is working, the brass can be pushed farther in by putting a plate on the shell holder and not engaging the rim. It also would not change the performance of the seating die to grind a little bit off the bottom so brass can be pushed in farther. These are only thoughts that have worked for me.
Craig, thank you, all excellent suggestions and impressive tinkering capabilities you have. Out of the box the brass fit perfectly into the rifle chambers. The two loaded rounds I made last night also fit perfectly, and it is clear that the neck area is especially close tolerance for a black powder gun. I learned this because I did fool with one of the case necks, just enough to raise some small bumps, and at first the round would not chamber perfectly. A gentle trip back to the sizer die did clear up those small bumps and now the live round chambers perfectly. I would also like to say that the Kynoch brass rims fit this gun perfectly. Perfect thickness. Kynoch does a good job of re-creating the brass of yesteryear, and though it is expensive, it does work well.
