I urge you to buy "Winchester Lever Legacy" by Clyde "Snookie" Williamson if memory serves me right... my copy is in my basement reloading room. It is a huge hardback reloading manual for every cartridge that one of the old Winchesters was chambered in... giving many smokeless powder loads even for the early 1873's, 1876's, 1886's plus other models.
Clyde & his friend developed their data using their own collections.. the .50 Express data was developed by using a deluxe takedown antique Winchester 1886... probably a $50,000 rifle. I paid $75.00 for our copy back in 2000... they can be found on Ebay & Amazon for similar or less cost now. If you load for any of the old Winchester lever rifles, you will enjoy this fine huge old manual, it has many great pix & also tells how the loads did in various game animals.
By the way, that is a dern nice old rifle, congrats on having it.