Actually, my Sharps is a carbine version with 26" (66 cm) barrel. Yes, the cowboy dies typically have a slightly larger expander in them.
You want to be sure to use a non-petroeum based lube with black powder. Petroleum based lubricants result in rapid formation of very hard fouling. That means frequent cleaning and is a real pain.
I have a lever action in 38-55 as well as my Sharps rifle. I can shoot a two-day match (75+ rounds) without cleaning. At the end, I can clean the bore with 4 patches.
The best cleaning solution I've found is what we term "Moose Milk". It is a 10:1 dilution of Ballistol in water (an oil/water emulsion). The water is what dissolves the black powder fouling, and when that evaporates, the mineral oil left behind lubricates the bore and provides corrosion protection.