Setting it during slavery times basically requires anachronisms--like most revolvers at that point had to be loaded similar to a muzzle loader (you loaded each chamber with the ball and powder separately), you had to put percussion caps on each chamber, too, I'm not sure if they had paper cartridges that sped up the loading process or not, but anyway, I can't remember if they got around this by just having them have a ton of extra revolvers on their persons or if they were loading anachronistic revolvers. Or if there were some revolvers at the time that were more like a modern revolver or even late 1800s type revolvers
EDIT: Like this was a common one of that time period:
http://en.wikipedia.org/wiki/Colt_Army_Model_1860
"Loading is a somewhat lengthy process, with each of the six chambers drilled into the revolving cylinder being loaded from the front, or "muzzle" end. A measured amount of black powder is poured into a chamber. Next a lead ball is placed at the opening of the chamber and seated by firmly pressing it in with the pivoting loading lever which is attached beneath the barrel of the revolver. For sealing each chamber, an over-size 0.454-inch-diameter (11.5 mm) lead ball is trimmed slightly by the loading ram to enter the chamber. Also, as well as for many 19th Century civilian shooters, modern shooters often place a lubricated wad between balls and powder, or, alternatively, pack lard or a commercially-sold bore lubricant at the mouth of each chamber to prevent powder in one chamber from being ignited when another is fired, which is known as a chainfire.
When the Colt Model 1860 was used by 19th century soldiers, they most often loaded the gun using paper cartridges. These cartridges consisted of a pre-measured load of black powder and a ball, wrapped in nitrated paper (paper that had been soaked in
potassium nitrate and then dried, to make it more flammable). To load each chamber, one only had to slip the cartridge into the front of the chamber and seat the ball with the loading lever ram. Then a percussion cap was placed onto the raised aperture, called a nipple, at the back end of the chamber."
The paper cartridge would make it faster but again, in that final shootout, did Django have like five revolvers or did he only fire two revolvers worth or did he load his revolvers quickly anachronistically?