The technology needed to make blackpowder weapons is rare, and pistols, handguns, blunderbusses, etc are prone to malfunctions, jams and misfires.
Each time you roll a 1 when rolling to hit with a blackpowder weapon (handgun, pistol, blunderbuss, warplock pistol, etc), roll a D6 and consult the blackpowder weapon misfires chart.