I have to say that during re-enactments misfires were rare, actually very rare. I used the 1853 Enfield Rifled Musket ( I was in the 32nd Virginia Infantry) and when they did occur it was always due to a faulty percussion cap. In the war of course other causes such as damp/wet powder or, if fired for prolonged periods, a blocked/fouled nipple or barrel could be responsible.
As most people know, it was possible for a well drilled infantryman to fire three rounds per minute, so the rate of fire wasn't actually that slow. I'm not sure misfires would bring anything to the table in this game, as the re-load time/rate of fire already presents a step away from the normal FPS type scenarios. Adding a further hindrance would, in my opinion, be a mistake.