It is a tapered down 45-90.Its not a bottleneck,but it has a lot of taper.
The 40-70 Sharps straight and 405 Win have a case head approx. like the 303 Brit and the 30-40.
If its an original old 1886 in decent shape its worth some money.Maybe enough money to not shoot it a lot. I ried to give an older gentleman an idea of how much his light weight octagon takedown checkered and color cased original was worth.I'm no expert,but IIRC the book was headed toward $10k.
That one was rather special.Plus,it had the provenance of being on the family ranch since purchased new.
Doing my research,it was strongly recommended that if the barrel did not say "Nickel Steel" that shooting jacketed ammo or even very hard cast was a bad idea.Apparently the steel was very soft and the rifling would not hold up to it.
In keeping with that idea,and assuming Pathfinder is giving good info,you'll perhaps have to experiment to find a bullet dia that will chamber.If cast of a fairly soft alloy,the "spank" of a BP charge may bump up the bullet to seal.
The Express Rifle concept was light bullets and "high" velocity.I think the 45-90 load for the 86 was a 300 gr bullet,or so. They were not twisted for heavier bullets.
I would not use pistol powder and fillers myself.Thats risky business.
There are some very slow powders that have been used for BP pressure loads. Accurate carried a few,maybe 5744? I hate to go by memory.
I'm sure Venturino wrote a book about shooting BP lever guns. Washing the brass so it doesn't turn green and cleaning the rifle properly make BP a high maintenance task but its probably among the best choices.If you form down 45-90 Starline,you may want a neck anneal. The early BP cartridges 38-40 and 44-40 are known for some taper,and very thin,soft necks.That served a purpose.Easy obturation,a good seal to keep the BP mess in the bore and not in the action. Detailed disassembly and thorough cleaning of an 1886 action is not for every horse bound travelor. .You might consider keeping your necks soft.If you size down 45-90,they might even need reaming thinner.
IF they are still in business,it might be worth a call to Mt Baldy Bullets. They make good stuff and they know the 1886.