I just ignored as best I could his historical mis-steps. The sad thing is that his using historically incorrect firearms doesn't add to the story. I think he does it just to PO us history buffs.
Will Ketchum
Mostly, I don't think he either knows the difference, or worse, doesn't care! Hollywood types usually don't care - they just think it looks "right" for the scene or character.
I've seen similar gaffs in other books - westerns, mysteries, suspense, etc. Sometimes I just keep reading, but a few times I've tossed the book and found something more to my liking. One of the worst was a novel about the 19th century Marine Corps but the author used WWII-era slang, ranks, strategy and tactics, etc. It became too much BS to swallow and I gave up less than a third through it.
At least I ain't alone in my opinion of McMurtry's books!