I would advise you to first go for this achievement:
This makes sure that you have enough MGR to trade for this achievement. However you only need about 300 MGR to be able to obtain this, depending on what ammo you are able to sell.
So go to any exchange shop. These are located on these levels: MARKET, ARMORY, DEFENSE, POLIS. Earlier levels also have these shops but you will definitely not have the required amount of bullets.
Now first sell all your normal ammo until you run out. Now use your MGR to keep buying shotgun ammo until you can't buy any more. Now sell the ammo again and than buy the ammo again. Now repeat until you get the achievement.
See it counts every bullet you exchange, even if you would have exchanged it before. This is why you can unlock the achievement with less than 500 rounds. If you did not unlock the achievement then you are still short on total MGR. Try selling your weapons and medkits/grenades if you are just short for some extra MGR.
After you get the achievement make sure to reload checkpoint so you will get all your MGR back as it's too valuable to trow away!