With a 3 or 5 groove, about all one can do is to measure max size on a lead slug bumped up inside the bore. That's worked OK for me in the past.

An over sized long-sided bullet might be the best bet due to the large bore. That is, unless Buffalo Bullet company or some other can provide a mould casting the correct size.

I'm not surprised about the 22" twist, actually as I don't remember reading otherwise. The old fellow at Hefley Creek Rendezvous who won several years in a row in the Quigley shoot with his 1884 Springfield was shooting the 500gr. 'Government' bullet. Trouble with that one from Lyman, is they sometimes cast undersized. He was shooting the battle sights - range about 750yards, but uphill at about a 30 degree angle. Actual range? - need a mathematician - that far is good enough for me as long as I get some sighters.

If at .459" and fairly soft 50:50 WW/Pure, they will probably slug up (obturate) and shoot well with real BP.