Sugar Colt (1966)

Near the end of the American Civil War, an entire unit of one hundred Union sharpshooters vanish into thin air in a rugged, mountainous region. To shed light on their mysterious disappearance, Sugar Colt, a former special agent, superb marksman, and director of a shooting academy, rides into the dusty town of Snake Valley, posing as Doctor Tom Cooper. But, newcomers, especially the brazen ones, are rarely treated with respect in the God-forsaken hamlet. Now, Tom will have to put his skills to good use, as guns have the final say.

IMDb: https://www.imdb.com/title/tt0061040