There Will Be Blood
 

I'm namely wondering about the gun he uses to kill his brother's imposter. I can't find any good screenshots, but Bandy, the man who's land he had been worrying about, found him with it and held it up clearly at one point.

Since it's a western drama, I'm going to assume it's the Single Action Army. I saw like literally five seconds of the movie and I thought that's what I saw. I'm not 100% though.

I seem to remember it being an 1877 Lightning but I havent watched it in almost a year.