AT LONG LAST AFTER 35 YEARS! i just found it today. It wasnt a movie. It was a color Episode of Wagon Train called the Charles Harrison Clarke Story.
Michael Rennie is a British journalist traveling with the Wagon Train. He is also a big game hunter and, I think, former soldier thats been in Africa. He has with him, what looks like, a .50 Sharps with a brass telescope. Even had 6 big cartridges on his belt.
Well, the wagon train gets surrounded by Kiowa and Comanches in an Adobe Walls like setting. Long story short, Michael Rennie, at the request of John McIntyre and Brian Keith, takes out the Comanche chief at long range in an amazing shot wih this .50 sharps.
They just aired it on Heros & Icons channel about 15 minutes ago prior to this post. I saw the last half of it and realized towards the very end THIS is what I saw decades ago as a kid.