The theory is that with a set you can swap the flintlock and touch-hole liner for the percussion lock and drum-with-nipple. First, unless you bought the percussion and the flintlock unit for swap, it's pretty tough to get the drum lined up so the hammer flatly engages the top of the nipple. Track sells a gadget that lines up the drill for adding the nipple to the drum which would be far quicker than repeated attempts to line up the drum with nipple already installed. Just shoot it as a flintlock and don't look back.