the zhentarim in my game are mass producing and stockpiling muskets

Sulfur, charcoal, and potassium nitrate (saltpeter) for gunpowder.

>guns
iron industry for barrel
mechanics industry for wheellock or flintlock
carpentry for stock and furnishing
>bullets
a way to make round lead balls of (relatively) consistent size
>gun powder
charcoal, sulfur, and saltpeter
the first two are easy-ish
but historically the last one is the hard bit, with france and the Confederate states of america resorting to distilling it from piss, and many countries can bottleneck your powder supply by squeezing your overseas trade

not related but training
while any old peasant can be trained to fire, endless drilling and practice was required to be useful, and professional mercenaries and later standing armies could make or break a battle
and a robust supply line
your troops will always run low on ammo, so there should be a lot of logistical support for anyone with a primarily arquebus armed army

File: Dueling_pistols.jpg (7MB, 3888x2592px) Image search: [Google]

7MB, 3888x2592px

Full set.

>>53980028
>>53980028
Historically, smooth bore muskets could also use stone balls when lead wasn't available
It's a lot cheaper, but also less accurate and has a shorter range
Stone balls also tend to shatter mid air (not good), but if they hit, will most likely shatter inside the target, punching fist-sized holes on their way out (think dum dum bullets) and shredding everything inside
On the off chance you survive that, they also leave hundreds of stone fragments embedded in the flesh, thus creating a lot of infection points
Because it was cheaper, less reliable and couldn't penetrate armor or protections as well as lead, it was mostly used by peasantry and poachers
Other downsides are that it's pretty labor intensive to grind a stone ball to the right dimensions and that because you can't really standardize the shape (at least if you do it by hand) you run the risk of damaging the gun, or having stray shots

>>53979980
They will need metallurgy and black smithing skills to make the barrel that can withstand the gunpowder exploding.

>>53981350
Also you need to make powder with a consistent burn rate. It's not just a case of mixing the chemicals together.