Speaking of movie mistakes:
In the Bogart movie, Sahara, the Krauts seem to be shooting '03 Spingfields. That cocking piece really stands out.
In the final gunfight of The Sons of Katie Elder, the Duke gets off about 13 shots from a single Colt without reloading.
In Unforgiven, a posse is heading out and someone says he's got to go to the store to get some .30-30s - quite a few years before they came out.
Of course, there are the "recoilless" rifles and pistols appearing on screen. One of my favorites is Charles Bronson firing slugs out of a short barrelled 12 gauge in The Mechanic with nary a trace of back thump. If you've fired a slug through a pump gun with a barrel like that, and I have, you'll appreciate how funny it is.
In Sergeant York, Gary Cooper carries a Springfield, when the real Alvin York carried a P17 Enfield, and the famous scene in which he's mowing down Germans with a Luger is wrong too. The real York used a .45. I will admit there was justification for the latter as it is apparently not possible to operate a 1911 with blanks. And, by the way, York was not yet a sergeant when the action took place; he was still a corporal. The promotion to sergeant came later.
Speaking of Open Range, I just love the huge hole Boss Spearman blasts through that wall with his shotgun, with no perceivable recoil, and how far back the guys he blasts with that scatter gun get thrown by the impact. I remember an even more absurd "hole in the wall by shotgun blast" in a James Bond movie, but I forget which one now, except that it starred Roger Moore. Bond and the girl who fired the shot (yeah - without recoil) are able to run through the hole and escape the bad guys.
And, I can't count the number of times I've seen Colt Single Action Armys in movies set years before 1873 - even during the Civil War.
I also remember a Robert Ludlum book sequence in which a character fires eight rapid shots from a .357 magnum revolver. Having read a number of Ludlum yarns, I can testify that the man doesn't know diddley squat about guns.
Finding gun goofs in movies and books is a kinda fun pastime.