Pathfinder. Descriptions for firearm discuss early black powder.

Using masterwork powder adds one die of damage to the standard damage caused by the weapon.  It also reduces the critial multiplier by one, minimum of x2.  This includes adding a dice of damage from explosions.  It also adds 2 to the chance of misfire, unless fired from a masterwork firearm, and using a masterwork firearm and bullet .  Using a masterwork firearm reduces the chance of misfire by one.  Use of a masterwork bullet in a non-masterwork firearm does not produce a reduction in misfire, but can increase to hit, like masterwork arrows for instance.