So, bumping this with an update...
The smith I took it to earlier this year...did nothing. He said all the measurements on the chambers were correct to the originals, and refused to do anything about the loading window because he wasn't comfortable with it. I'm not pleased about that.
So I tried shooting with 25 grain cartridges, not long after, and THOSE wouldn't seat all the way, either! However those were being just as big of a pain to fit into the loading window as the 30 grain, so I figured that was likely the problem because I couldn't get them to start straight.
After some searching once I had time I took my gun in to a different smith last month to have both issues looked at. Although they were able to enlarge the loading window so now the 25 grain cartridges fit properly under the lever, they weren't able to do anything about the chamber itself because of the grooves for the ratchet/lock mechanism, (drilling out the back of the chambers would make the metal too thin) so they couldn't make it take the 30 grain cartridges.
Whatever, I still had plenty of 25 grain made up, so today I went back to the range...
...and even though I can now start them level, the damn gun STILL won't seat the 25 grain cartridges all the way.
I honestly don't know what to do anymore. My powder charge is already lighter than what the original cartridges were, and the damn things still won't fit.
I'm using the nitrated paper from Buffalo Arms. Could it be that the paper is too thick? I had flash paper, which is much thinner, however it doesn't keep safely, and I just had to dispose of a whole package that was going bad. Is there anyone else who makes nitrated paper with thinner sheets?
Or do I have to reduce my powder load even FURTHER, even though it's already well below historical loads? Could there be a difference between modern black powder and what was available then that could account for this (I'm using Goex 3F)? Like maybe the older powder compressed more readily?
I also noticed the paper isn't burning up completely like it's supposed to, but leaving rings in the chambers, which obviously doesn't help loading subsequent rounds (the ones I tested at home this morning to test the fit of the window ALMOST seated completely). I'm currently using silicone silicate to hold everything together as on the historical cartridges, but should I be using something else?
I'm just about at the end of my rope with trying to get this to work.