Hello WesDurn,
I too bought the 1876 rifle, and I have used both the Lyman bullet and the RCBS bullet. I had the best result with the RCBS bullet, and I used Starlight .45-70 cases that I cut down. The first time I did this, I cut the cases to the original length of the .45-60 brass. When I loaded these rounds into my rifle, they would not cycle. The short version is that they were to long. I seated the RCBS bullet to the crimp grove on the bullet, and this made the loaded round to long. When I used the RCBS bullet, I had to cut the cases to 1.84". After I cut the cases to this length, and seated my bullets to the crimp grove on the bullet, my rounds worked just fine. Cut the brass to 1.84", and load the round to an over all length of 2.27". The only other issue I have had is that I have had to re-anneal the brass. After about 3 loadings, the cases started to split. Good luck, I think you will really like the 1876 rifle. Mine shot to point of aim, right out of the box with my smokeless loads. Black powder, that is a different story...