I've read that the reason they used Trapdoor Springfields in some scenes in The Outlaw Josey Wales was because its safer to fire a blank cartridge than to fire a blank-loaded muzzle loader. I guess maybe the blank cartridge doesn't need a wad or as much of one because it can be crimped? Anyway, if you look closely you'll see that they are shooting Trapdoor Springfields from inside the house.
The still pictures and closeups show a regular Colt Walker and the other, lesser, revolvers but the shooting scenes were done with revolvers converted to fire cartridges for the same reason.
That same issue may have had something to do with the prevalence of guns from the future in so many other movies as well.
