I am using the Firearms in Freeport article as written. I think it is reasonable to make it an exotic weapon proficiency. I consider these items "blackpowder weapons" which mean that there is no set "cartridge" to load...
Given a rag, a horn of blackpowder, a bag of pellets, and a ramrod, could you load a blackpowder rifle correctly, having perhaps only seen one in passing? It involves getting the right size ball, and the correct amount of powder in the barrel in the right order and at the correct pressure so that they are expelled under pressure, and not so tight that the gun barrel explodes instead!
Besides, they are not unbalancing and in fact quite limited in practicality. Wet powder won't fire, and dry powder is quite unstable, meaning a PC carrying it will take damage from exploding powder if they are caught by a fire spell or effect....
My players keep saying I have a taste for blood...I'll answer them as soon as I'm done with my steak tartare...