It would make a lot of sense to me if some NPCs (I am thinking specifically of the guard NPCs topside which have similar numbers and factional distribution compared to the gangs) would buy ammunition every week from players in the same style as others buy drugs. The two systems aren't identical (there's several types of drug creation tools and only one ammo tool, ammunition creation can't explode as far as I know) but I think these are basically a question of tuning and you could start the payout values lower and scale them up until there was some kind of equilibrium.
One of the cool aspects of having something like this is, while it increases the potential for income generation of munitions characters a lot, it doesn't actually increase the size of the economy at all because the total amount of automated income that can be earned from all players remain the same.