Due to the Machine Loot introduced in the June’s Update,
You can get about 80% of wasted ammo back from tanks/harvs. In other words, if you use .50 cal to take them out, you get .50 cal ammo back from them. For example, yesterday i took FNIX tank out with my 6* .50 cal and got about 45x rounds of .50 cal ammo from it. Of course, i used about 15x rounds to take it out, leaving me about 30x rounds profit.
Another method of gaining .50 cal (or any ammo) is using Ammo Packs. Each ammo pack has 3x uses and it gives the full clip of ammo of whichever weapon you are currently holding. E.g hold .50 cal with AP ammo loaded into it and you get 10x rounds times 3x uses = 30x rounds of .50 cal AP ammo from it.