Sharps Rifle.....1500 yard kill?

[meta]

I've found myself in an argument over whether there is any truth to the legend that Billy Dixon killed an indian at 1538 yards with a Sharps .50-90 rifle. I know the trajectory is absurd, and I know the chances of hitting remote at best, but can anyone run this over ballistically and give me the nuts and bolts? AKA: DUBUNK THIS BS. Here is the article:
http://the_montanan.tripod.com/Sharps.html

[BountyHunter]

Hope you have some salt and pepper handy for that crow.

Go to "Precision Shooting at 1000 Yards by Dave Brennan" and sold by Sinclair International www.sinclairintl.com. One of the last stories is about them "replicating the famous Billie Dixon shot".

Do not have the article in front of me, but if I remember right they put 3-5 shots on plywood siloutte of horse and rider at the distances quoted in the story.

Also the Confederates used a muzzleloading Whitworth rifle to get 1000-2000 yard hits.

Not common but skilled marksman have pulled it off.

BH