 FACT: At the end of the movie, when the 54th is marching onto the  beach preparing to assault Fort Wagner, about a dozen cannon are  present, with caissons directly behind them. In reality, caissons,  which contain large amounts of ammunition, were usually kept about  50 yards behind the gun. That way, if a caisson was struck by an  enemy shell, it wouldnt explode and destroying nearby artillerymen,  cannons, and other caissons.