Appreciate your point ,but I thought these conversionscame after the civil war.The use of cartridges in the filmI think at this period is a bit of artistic licence.Wild Bill Hickok was using cap and ball revolvers wellinto the 1870's.
PS, An interesting real life example of this is the recent archeological findings at the Custer battlefield, the cavalry was equipped with single shot rifles, the Souix had superior Henry Repeating Rifles, and they concluded that rather than the popular notion of a "last stand" the battle was over quickly.