From the history of those times that I've read, if one shootin' irons worth of lead wasn't going to be enough, the usual procedure was to carry more loaded guns.
Some carried up to six revolvers, four on their person and two on the horse.
But then, as now, serious shootists did things differently.
Having extra loaded cylinders would seem to be a natural solution.
If we can think of it, no doubt so did they.
Lock the doors, they're coming in the windows.