The correct answer is whatever they had to fit down the barrel. But historically it was buckshot type shot. Was what I read in the past was they used pre-rolled undersized paper cartridges rip one end, dump powder, rest goes in on top of it shove down and the rolled paper holds shot in place - prime and fire. The author stated it was close range firefights with a moving coach being guarded from horse mounted outlaws. It was not so much about accuracy as most shots were close it was about rate of fire, and getting a quick spread with lead to make up for accuracy. This was why many coach guns had shorter or cutoff barrels. They were easier to faster to load on the move, easier and faster to shoot, and shot spread quicker. Hope this helps. If I run across the book I'll share details with ya. But I been out of the cowboy shooting stuff for a while now.