don't know how to make a firearm

Ranged item element intensifies

XD 
But keep in mind that bow-like charging effect animation and damage scaling isn't possible without Java knowledge :D

 Fortcraft mod intensifies

You can make an item with a "when right clicked on air" procedure. 

Make a ranged item called bullet and go to the bullet section.

Make a model for the bullet and a texture, save it and go back to the procedure.

In the procedure, add the "Shoot ranged projectile as () type of (bullet)".

That is pretty much all of it.