Here is the scoop on Quigley's rifle from the DVD. The scene is when he first gets to Marston's ranch, right before he shoots the bucket.
Marston: "Aaahh, the legendary Sharps. "
Quigley: "You know your weapons. It's a lever action breech loader, usual barrel length is 30 inches, this one has an extra 4. It's converted to use a special . 45 caliber 110 grain metal cartridge with a 540 grain paper patch bullet. Fitted with double set triggers, and a vernier sight. It's marked up to 1200 yards. This one shoots a might further. "
Marston: "An experimental weapon, with experimental ammunition. "
Quigley: "You could call it that. "
Later when he is talking with the shopkeeper about reloads, he says you can substitute a . 450 #2 British musket lead.