"Warning Shot"
My favorite is an old TV movie where the whole premise of the movie is a howler. It starred David Janssen and was called "Warning Shot". Janssen's character was in big trouble because some witnesses to his shooting a prep falsely claimed that he never fired the "legally required warning shot." the whole movie was about his trying to prove that he actually fired a shot in the air before killing the perp--which would, of course, get you fired or at least laughed out of the squadroom in real life. I guess someone figured out that it was based on baloney, because I've never seen it since the first time, even on late-night cable.
Stephen King used to be awful--he did the revolver-with-safety thing in at least 3 books, and once repeatedly used "rifle" and "shotgun" as synonyms--but in "Cell" he seemed to be trying harder.
It's not a firearm error, but I once saw an episode of Miami Vice where the detectives were chasing a guy in a Lamborghini Diablo; they were driving a stretch Lincoln limo--AND THEY CAUGHT HIM! I never got into that show, anyway. If a vice cop is driving a Ferrari, wearing Armani suits and living on a yacht, there's something WRONG, man...