Corrected entry: The Union Soldiers use a Gatling gun to attack the bushwhackers near the beginning of the film. The Gatling gun was developed, but never used in combat during the Civil War, especially not in the less important Trans-Mississippi theater. Furthermore the Gatling gun shown has 10-12 barrels, the first one developed only had 6.
Corrected entry: In the beginning Frank is shooting a lever-action rifle. It's supposed to be Civil War era. The gun is actually a Henry Repeating Golden Boy produced a few years ago.
Correction: The rifle Frank uses is a Henry model 1860 lever action rifle, which was very much Civil War-era.
Corrected entry: Jesse James's gang wasn't called the James Gang but the James and Younger Gang.
Correction: When Jesse James and Cole Younger are robbing the first bank, someone asks who they are, and Jesse answers "the James Gang." Cole objects and they decide to call it the James-Younger Gang. Also later in the movie they dispute calling it the James-Younger or Younger-James gang.
Correction: False. They were used at the siege of Vicksburg, etc.