Mike Venturino and some friends tried the long range (1538 yards) Dixon shot. They found it was quite possible; it just amounts to getting the elevation right with the old Sharps!
By the way, they researched the site at Adobe Walls and determined which guns/cartridges were used in the close up phase of the battle, before the Comanche withdrew initially.
Mike has a whole chapter on this Adobe Walls thing in his, "Shooting Buffalo Rifles of the Old West", available from MLV Enterprises, P.O. Box 914, Livingston, MT 59047. Price is about $30 and worth it. All of his books are excellent; ask for a list and prices.
Lone Star