The film is set in 1862 but features Gatling guns that, presumably, formed part of the fort's arsenal when it was in Union hands.The gun was designed by Dr Richard J Gatling in 1861 and patented on November 4, 1862. Though two examples were employed near Petersburg and eight fitted on gunboats, it was not accepted by the American Army until 1866. So it's most improbable that it would have been available in a remote theatre of the Civil War.