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.