The population in the USA in 1870 was under 40 million people, about 10% what it is now, but it was a violent place outside of the West. There was violent labor and racial strife all over the country. Manhattan in NYC is now one of the safest places in the country, but through much of the 19th Century it was controlled by gangs. Vigilantes took over San Francisco because the criminal element was out of control. Extra judicial punishments like lynching were common, as were state executions.

Most of the Old West was settled primarily by men; railroad workers, miners, hunters, cowboys, loggers, and soldiers. Their recreation was primarily drinking, gambling, and prostitution which was practiced openly. All of those activities are related to violence. There was also violent fighting over land, and not just with the Indians who were warriors after all.

I don't see civilians walking around with AR-15s slung over their shoulder. If the assault gun ban passes they will be worth their weight in gold, so there are additional reasons besides security why people buy them.