They have had guns in the US for many years it's true, but I suspect you might have a problem conducting a mass shooting with a muzzle loader.
They had fully automatic weapons in the twenties, and most American men, certainly those who served in WW1, would be familiar with them. And I don't know of anything like what happened at Sandy Hook or Columbine until the eighties. I think the texas university sniper kicked off the modern age of mass shootings in the mid sixties.