Well, if you go with use-activated major creation, that would be 90,000 gp, 9th level caster, prereqs of major creation and craft magic arms and armor. Now, if you are using the renaissance firearms in the DMG, you are getting up to an extra 19.5 damage per round (if enchanting a rifle). An equivalent would be the flaming weapon enhancement, at 3.5 for +1 market price modifier. It would take +5 to +6 in such enhancements to get 19.5 damage. That makes this look overpriced. I'd say more like 70,000 gp. Of course, the more modern a weapon you are enchanting, the less useful this enchantment becomes, and the less it should cost. It's nice not to have to change clips with an automatic pistol, but it doesn't give you anywhere near the same advantage.
