This film is set in 1901, when double action revolvers were definitely available. Books. however, had been shooting single action pistols for decades and continued to use single action models, which had arguably a lower rate of accurate fire, because "fanning" a single action pistol could change the direction of fire (I am talking about real pistols, not video games). At close range that might be good enough, but the farther away the target, the less likely it is to be accurate. But maybe if I had his skill, I might prefer a single action weapon, because against a single opponent, one good hit will do it.