Nobody has ever looked for the lost guns of the Custer Battle as far as I can tell. I have some ideas about where to look. The indians who surrendered to reservations in 1876-77 had to surrender firearms. They wouldn't be foolish enough to surrender captured Custer guns. There was near-universal fear among the indians of admitting to the white man their presence at the battle.
The indians were armed with a variety of arms at the battle. Some had Henry's and Winchester repeating rifles which were superior in firepower to the cavalry Springfields. I can see the guns would quickly have been considered "radioactive" and something to hide.
For a number of practical reasons, I believe the guns were hidden within a two-day ride of the battle.
Well, you may start a run on the place, much like the lost mine at Superstition Mountain! :)