A gunsmith isn't just about pistols, but any firearm. You can perform work that a regular blacksmith would find astounding. Your tools allow you to create special bullets, cannons, and all the working parts of a firearm. You require Blacksmithing to take this skill. You or an assistant must have the appropriate weapon skill to get bonuses to your creations. This skill covers the use of plastics in complex cutting-edge weaponry at the highest levels.
In ancient times, the best you could do is fireworks, primitive cannons, and perhaps flintlock rifles if history allows for it. In the future, weapons may rely on energy propulsion and other techniques which require a good understanding of electric systems and technology.