Arquebusiers are ranged combat units. To create them, give a settler an arquebus. Arquebuses are primitive firearms used for mid-range combat. They are expensive to create, with arquebuses using large amounts of iron, along with advanced firing mechanisms. They are also expensive to use, with gunpowder requiring sulfur (found in ore near volcanic regions), charcoal (produced from wood, charcoal is also very good for use as fuel, as it is more energy dense than an equivalent amount of firewood), and nitre (found in ore near arid regions). They also require a gunsmith to manufacture. Gunsmiths are specialized blacksmiths dedicated to producing firearms and its respective ammunition.
Arquebusiers don’t benefit as much from experience than Archers do, but also don’t need as much skill to operate effectively as Archers do. This allows you to quickly create a militia of unskilled infantry so long as you have the industrial power to supply guns and ammunition.
Arquebusiers have greater damage potential than Archers. They, however, reload slowly (not to mention can’t move while reloading) and have poor accuracy at long ranges, which gives them poor damage application. Melee units slaughter them at close range and Archery units slaughter them at long range. At medium range, however, they are deadly. They also have high armor penetration, which makes them a formidable adversary for sluggish armored units. Arquebusiers themselves have little armor, which makes them more vulnerable to arrows and bolts as well as unarmed melee and knives.
Pistols - Small firearms. They have the worst range of any ranged weapon, and reload almost as slowly as arquebuses (and also less damage and penetration). They are, however, useful sidearms to use when cornered, and useful primary weapons for the insane or insanely skilled.
Rotary Musket - Large, slow, defensive installations. There are two ways to move them: repackage them and move them with any ordinary unit (unpacking them takes a lot of time, use for long distances or in rough terrain), or have the gunner wheel them around (movement is slow, but no unpacking is necessary, use for short distances or in hostile territory). Rotary Muskets are essentially like arquebuses mounted on a wagon, but with better damage and accuracy. They have three barrels, which allow them to fire three times before reloading. This also, however, means they must reload three times afterwards. Only units that have experience with firearms can use the Rotary Musket.