make them ammo neutral? as most zip guns are pretty vaguely sized, or prehaps not an actual ammo, just runs on a "cartridge" made by anyone, 1 gunpowder and junk or metal parts makes hundreds.
Nope, better no Gunpodwer. So you can shoot with your first gun without 60% science skill.
Wood+N Junk(s) for zipgun.
N Junk(s) for ammo.
So you still need 40% repair (or 30%... never got characters with so low repair.... so don't even know) for junk.
Or add repeatable quests for noobs and Gunpowder as reward, but make it take more time, than harvesting minerals, so it can't be abusable.
Also if you don't like idea without Gunpowder, 'cause Zip Gun uses 9/10 mm, you can make as SG primitive weapon next:
http://fallout.wikia.com/wiki/Spear_GunSpear Gun.
So you don't need god crafting skills for this.
Just Wood and some Junk for gun.
Ammo from wood only...