Basic Oil Processing has the perfect output ratio for Flamethrower ammo. So you can just stick a Chemical Plant on the output of a refinery and give it iron. The refinery will starve the plant, but who cares? It's personal ammo production.
I added it to the wiki: https://wiki.factorio.com/Flamethrower_ammo
![Image](https://wiki.factorio.com/images/thumb/Perfect_Flamethrower_Ammo.png/300px-Perfect_Flamethrower_Ammo.png)