Common usage began only after production of reliable caps was sufficient to allow an adequate long term supply out West. I think its safe to use mid-40s when talking about Common Usage in the West.
Remember, everything that was used out there was carried on foot, horseback, or on a few steamships, and wagons. That included caps, flints, powder and lead. Flints could be made when certain kinds of rocks were found, in the West, when needed, and supplies ran low, but caps had to be brought from an Easter supplier, often 1,000 miles or more away. Its hard, today, to really understand what those kinds of distances mean, when you don't have cars, trucks, railroads, or airplanes, and no airports, RR terminals, or Interstate highways to get you there.