With the coming of gunpowder, a new age dawns on the world...
In around the 1400s and up until the late 1500s, gunpowder becomes more prominent in Europe and the Middle East. However firearms were used in China starting with the introduction of the fire lance in the 900s, and the first "hand cannons" started showing up around the 1200s - 1300s. Cannons are also included on sea vessels as well as castle fortifications, and become more common in militaries.
In addition to the inclusion of new technologies, the rediscovery of the Americas occurred around 1492 and spread throughout Europe after he returned in 1493.
In the 1400s and onward, firearms are becoming more common as the weapon of choice in warfare although martial combat and even bows aren't being made obsolete yet by the progression of gunpowder technology. These firearms are powerful but take time to reload, as well as becoming soiled in unfavorable conditions (usually becoming wet or otherwise soiled).
Firearms are their own class of weapon, the weapon skill being Weapon (Firearm). This covers all firearms; pistols, rifles, shotguns, although cannons are not covered under this as they are considered siege artillery.