Happened during year 1, folks weren’t happy that machines/lootables didn’t drop ammo that correlated to what they were using, it was a big overhaul actually.
@OBiW4NSHiNOBi there is regional loot pools as well. But it’s not “more” weighted towards the region, that just is the overall list in the item pool. the way i figure it goes is;
regional
lootable
priority
So say you’re in region archipelago but you fight hunter that has 7.62 ap in it loot table, you can still find AP ammo in that loot table even if it is missing from the region, but then when searching ammo boxes because that pulls directly from the region loot you won’t find AP rounds.
As @tene already explained, it came on 1st year. To be specific, in June '19 update, part of a “Loot Containers” and “Machine Loot”,
link: June Update Now Live | Generation Zero
For example: .44 ammo.
If you don’t have .44 Magnus in your inventory, you still can find it at some ammo crates, albeit rarely. Also FNIX hunters have that ammo in their loot pool as well, as primary ammo. But the ammo amounts you find, even with Salvage lvl2, are roughly 20x rounds.
Now, if you were to put .44 Magnus in your inventory (doesn’t matter if equipped or even holding it in the hand), you will start finding a lot more .44 ammo in the ammo crates. Also, FNIX hunters may have more ammo per pickup.
I started a fresh savegame on adventurer to test exactly this, put the AT-WAD in my backpack and used vanilla weapons to destroy enemies. At the time I crossed to border between Archipelago and Farmlands I had more then 500 FMJ and more then 200 AP. I just wanted to have the difference between my actual playthrough, were I scrapped the all the 3* DLC weapons in the Plundra. In that case I just had about 200 FMJ and a couple of AP.
This might actually just some RNG but to me the difference looked quite significant. So I’d say there is a difference although it is not that significant to recognize it easily.