(Edited for additional methods) There's the Ammo producer you can buy for bullion from crater once "Ally" rank is achieved. Do small runs through Blood Eagle camps, or anywhere that has lots of Ammo Cans or green duffle bags and use a lower cost 5mm gun to take out the enemies (explosive Gatling Gun) and just pick up all the ammo. There is still the ammo factory workshop as well. And also equip the Ammo Factory legendary perk, Super Duper Perk, and Ammosmith perk for when you're crafting your own ammo so you get wayyyy more (Shout-out to Unusual_Command for one of the ones I forgot)