I gave up on period correct firearms in movies, for the most part, long ago. I will say that many percussion pistols were modified to take cartridges, so the bandoliers are not entirely incorrect if the pistol is modified. Anyway, BP was around in reality until around 1900-06. (If I recall in Pale Rider, preacher changed cylinders, not cartridges. This was done in the CW, particularly by Forrest's troopers.)
All of the early rifles and pistols, Colt 45 Peacemaker, early Winchester lever actions, SW SChofield, etc were BP firearms. Even the early 1903 Springfield Rifles were BP firearms. There is an advisory against modern ammo on low serial number 03 Springfields. Remember the caution taught at firearms class to have old guns checked by competent gunsmith? This a good part of what they are wanting checked: to see if it can handle smokeless ammo.