I almost wonder if TFP saw something akin to what I did in my last A20 playthrough where I used nothing but my 2 junk turrets with AP ammo and cleared blood moon hordes with ease, even being on insane difficulty. It was costly, though; around 5k rounds of ammo used by those things which was around 30k iron. I suppose that's cheap by late game standards but that's still a LOT of iron. Now it has lead attached to it as a cost...at that point we just make AP ammo for our own guns and some explosives.
