Major Reno is shown using a Henry Repeating Rifle, which later switches to a Winchester 1866. No Officers were using lever action rifles at the Little Bighorn. They mostly carried Sporting versions of the standard issue rifle, which was the Springfield Model 1873 "Trapdoor" Rifle. George Armstrong Custer carried a Remington Rolling Block Rifle in .50/70 caliber.
Soldiers are seen wearing suspenders along with Major Reno, when he goes into the timber. There is no evidence that any soldiers wore suspenders on June 25th, 1876, and they weren't standard issue until 1883.
The Fetterman Massacre appears to occur in the summer. In reality, the fight occurred on December 21st 1866. Also, Captain Fetterman and Captain Brown did not shoot each other near the end of the fight. Fetterman was believed to be killed by American Horse.
George Armstrong Custer was not shot at the river. His wound to the chest would've been fatal. He was found at the top of the knoll on Last Stand Hill with shell casings from his rifle found under his body. His head wound either came first, which matches the account of Private Jacob Adams, who claimed that there was blood coming from the head and none from the chest, or the chest wound was first and the head wound could've been a post-mortem Cou DE Grace delivered by his brother Tom, or by Lt. W.W. Cooke near the end of the battle.
Crazy Horse's horse wears iron shoes (visible during the vision quest scene)