Brandon Lee, son of Bruce Lee was killed in a similar incident back in 1993.
They used dummy rounds that have the actual bullet, no powder, but real primers.
So close ups make the revolver look like its really loaded.
The gun was fired with just the primer which pushed the bullet into the barrel. When the replaced the dummy rounds with blanks and fired it, the charge was enough to fire the bullet in the barrel which struck and killed him.
If this movie was a western, its likey the same situation. That would explain the death but how do you wound the second person?