Another problem is the excessive force one need to pull a bow~ Many people do forget that but ever pulled an english longbow? Heck you know what a workout is if you then fire more than 20 arrow in a row~

Either way a flintlock is usually made of wood and metal. The barrel is naturally made of metal as well as the mechanism, the wooden parts are then the shaft as well as the decoration. If they are made of metal or not does not matter at that early state of technology.

I do give the point of a very low reloading time but here you can have multiple guns or more barreled guns~
On the other hand we now have an Green arrow like image of an archer.... well the problem here is the kind of technology. As far as i have seen it the only gas being used was by Cesar and other chemical compounds would be highly unlikely.... people do tend to forget that such things are needing ingredients.
Where do you get such? I mean each island is different from the one before.....and then when does he has time to create them? And if you are using gas...it could also hit your friends instead of foes, a sticky liquid needs first to be produced etc. etc.

Look at Usopp, he was using foul eggs and strange inflammable beans before, one could easily create them considering that nearly every dried seed has the same effect!
Lycopodium is rather common if you know that one but using such tricks on a bow? It is difficult, sure the arrow + rope trick works...also i think small glass-phioles are working which would then break on impact and so release something...but...the problem is how to create them? how to keep em flowing? 

Seeds can be planted....but small pices of wood to craft the arrows, the steel heads.....it is more complicated.....
Bullets...well bullets can be purchased everywhere i guess, marines as well as pirates, sailors and merchant are needing thse but in One Piece i never saw someone selling arrows or using a bow....unless it was a snake....