I have been an amateur historian for years and the period I have always been most interested in was the post-Civil War American West. I have read most everything printed on that era and the guns used then.
For 9 years I was active in the Cowboy Action Shooting game and Living History re-enactments and shot original 19th Century firearms loaded with black powder cartridges.
One of the things I learned is that the old saying "Times change, people don't." is true. Back in the 1870s, 1880s, 1890s and early 20th Century people were much like we are today. There were those who had no interest or use for guns, those who owned a gun or two but were not firearms enthusiasts, and "gun cranks" like those of us here. This last group might include some "professionsals" to include both lawmen and outlaws.
The people with no interest probably didn't own guns, just like today. Many more people owned guns for occasional hunting or maybe home defense but weren't connoiseurs, just like today. Among that group there were probably more shotguns, rifles and small caliber hand guns. The final group would own several and they would be the best they could afford, just like today.
The last group is where my research indicated you would find the SAA Colt. Yes there were S&Ws, Remingtons and M&H revolvers. I have owned and shot all of them and can tell you the one reason Colt dominated among the "professionals" and also why it was adopted buy the military.
We're talking about the black powder cartridge era. Black powder fouling was a big problem, particularly in extended shooting. The Colt SAA, because if it's simple design holds up better than the more complicated S&W, the poorer design of the Remington, or the fine machining of the M&H. Among serious handgun users the Colt SAA in 45 Colt dominated.
For much the same reasons, reliability and durability, the Winchester repeaters (Henry, 1866, 1873, 1876, 1886 and 1892) were dominate long guns with the same professionals. And yes, 10 and 12 gage shotguns were also popular fighting guns.