It's a 1863 Remington new model army conversion. As in converted into a cartridge gun.
http://www.flickr.com/photos/joelogon/5778840128/
http://www.thefirearmblog.com/blog/2...1858-revolver/
Some original remington conversions were made so that you had to remove the cylinder to reload. Sort of like an R&D conversion cylinder.
http://www.taylorsfirearms.com/cartr...cylinders.html
Clint just used extra cylinders.