Honestly, I'd go fo historical accuracy with this one. If it's possible to edit the chance of a weapon sapwning do so. Make shotguns and repeaters far less then a regular chassepot musket.
Double-barreled Shotgun: 5%
Spencer Repeater: 10%
Chassepot Musket: 70%
Remington Revolver: 15%
Or, you could simply make it selectable.