I have a Cimarron/Uberti 1876 in 45-60 with a 28 inch barrel. I think the twist is 22 inches and the bore is .458 inches. I managed to make it to the range today with my chrono and thought I would post some results. The wind was gusting way too hard for any accuracy but I was more interested in the velocity anyway. First my BP loading procedure. I weigh out each charge then trickle it through a 30 inch drop tube. Since I was using a gas check bullet I did not use a card wad. Using Swiss 1 1/2 powder I first tried 60 grains but it did not fill the case enough to allow for compression. I have found in my 20 years of BPCR shooting that most BP likes some compression with GOEX wanting the most and Swiss wanting less. With 65 grains of Swiss I was able to get about .070 inches of compression which worked well.
Load details: 45-60 Winchester
W-W cases that had been shortened to 1.875 inches and the rim thickness reduced to less than .062 inches
65.0 gr Swiss 1 1/2 through 30 inch drop tube
Winchester WLR primers
321 gr RCBS GC 1:20 alloy sized .459 SPG lube crimped in crimping groove
Crimp after seating
OAL 2.295 will feed fine in my rifle
Average velocity 10 feet from muzzle: 1364 fps
Comparison velocity for heathen powder
Same case, primer, bullet and OAL
28.0 gr H4198 + Puff-Lon filler velocity: 1342 fps
26.2 gr 5744 no filler velocity: 1210 fps
These loads are safe in my new Uberti replica. Your rifle may respond differently. My suggestion is to always determine the velocity of your max BP load for a given bullet then stay below that velocity when using heathen powder with that bullet.