Fallout – How do unlimmited-ammo weapons work

You can craft ammo with the Contraptions Workshop DLC using the Ammunition Plant.

I have yet to try this in-game myself, so this answer isn't very detailed, but I'm surprised no one has written a nice answer about it yet, so let me at least note it.

At low levels below 30 you will have a hard time locating ammo. Your best bet is to stick to areas near Sanctuary and search for ammo there. I am currently over level 50 and I have found that the easiest ammo to find are the ones used by pipe guns. The .38 ammo and an explosive or wounding automatic pipe weapon and you have a pretty powerful automatic weapon. I use a .38 automatic pipe pistol to mow down feral ghouls and it is pretty effective in crippling them. I currently have over 5k of this ammo and it is still increasing instead of decreasing even though I use it all the time. A stealth silenced headshot with the explosive version will kill mostly with one attack in vats even if the opponent is a legendary super mutant. The next easiest ammo to find is the 10mm which is used by the 10mm pistol and the deliverer. Next is the .45 ammo which is used by pray n spray tommy gun which I love to use because at maximum upgrade can use 100 bullets per reload and the bullets explode on impact.

Ammo does not weigh anything so pick up every single one you can find. Later on you will see that you have too much of these and you can use them as currency. The higher tier ammo will show up more as you level beyond 20.

Now where to get these ammo:

• Always search dead bodies. Everytime you pick up a weapon from a dead body you add that weapons ammo to your own. Just drop it afterwards if it encumbers you and you will still keep the ammo. Even dead feral ghouls sometimes have ammo.

• Ammo boxes, footlockers and duffle bags (the green ones lying around buildings.) Near "abernathy" farm to the north there is a camper where there is one and further north there is a power line tower with a duffle bag. Also near "abernathy" farms to the south there is a power line tower and there are 2 ammo boxes at one of the base of the tower. There are also a lot of those inside army bases. If you have been to the "USAF station olivia" there is a green ammo boxes and a yellow box around the crashed vertibird and also inside the station itself are a lot of containers with ammo. If you see one of those green military dividers they usually have a green ammo box near them. Raiders would also usually have green ammo boxes just sitting around the walls of their fortifications.

• Junk and weapon vendors sell ammo and the best location to get some is at goodneighbor or at diamond city. There are also wandering vendors like Carla and Cricket that sell ammo.

• Check every container. From every box to every cabinet to every safe. You don't know what you are going to get.

• The scrounger perk will also help with getting more ammo. According to the wiki affects: .308 rounds .38 rounds .44 rounds .45 rounds Mini nukes 5.56mm rounds 5mm rounds Fusion cores .50 caliber rounds Oddly, .50 caliber rounds tend to spawn the most often, making this perk very useful for Hunting rifle users. 2mm EC (patch 1.4) Fusion cells (patch 1.4) Plasma cartridges. (patch 1.4) http://fallout.wikia.com/wiki/Scrounger

To conserve your ammo usage please use different guns and different tactics for each situations Like for example:

• Sniping a group of enemies far off? The best sniper weapon is either the .50 or .308 weapons at low levels. Stick to one until you get more of the other ammo. The .50 is easier to get at higher levels. Always mod with suppressors to lower the chance of them locating you. Take a shot then move away and hide. Then rinse and repeat. Legendaries with two-shot, violent, instigating or mighty will do wonders to instant kill enemies. Do not do this with scorpions or mole rats they will dig towards you unless you are on top of a ramp or building. As a rule I don't snipe until I am on high ground or somewhere obscured. A high perception stat helps greatly when shooting using VATS.

• Need to mow down a lot of enemies? Use an automatic weapon like an assault rifle or an automatic pipe weapon. If you have ammo to spare for the minigun use that instead. Legendary weapons with explosive, staggering, furious, kneecapper and crippling can help take down multiple enemies. Aim for the legs when facing feral ghouls to break off their legs so they can no longer run towards you. Then when they are all lying on the ground just bash their heads in with any melee weapon you have. Also grenades are very effective for times like these.

• Short range and lots of enemies? Use the shotgun to take down enemies that are near. The shotgun with its spread shot will damage enemies heavily when hit. Legendary with wounding, staggering, furious, kneecapper and crippling are great for these situations.

• Always have a backup melee weapon.

Another way to get ammo: http://fallout.wikia.com/wiki/Ammunition_plant