They changed it from the book. The book (by Cormac McCarthy) is probably the bleakest Pulitzer Winner, let alone Oprah Bookclub (!) selection in a long time.
In the book, the revolver that the Man has is described as small, nickel plated and .38 (implied Special). I don't remember if it was supposed to be a six shot or five shot.
The description in the book made it sound like it was definitely a snubby. Perhaps a nickel 10, 15, or Colt Detective Special if a six shot or a Model 36 if it was a J frame.
The movie has been sitting in the can for over a year. It was supposed to come out last fall and the release date has been moved several times. That usually isn't a good sign.