As you collect Military rounds you will either use them as bullets or save them to buy ammo and supply's. Once you are within the safe areas of the metro make your way to an Exchange Kiosk. At some point you will need to buy ammo, this achievement should come over time. If you don't use the kiosks on a regular basis, save your military rounds and make several large exchanges.
Polis Quick Exchange Method
This will be the last place where you can exchange ammunition and earn the achievement. You can do this earlier on but that's down to the players digression
When you enter a Exchange Kiosk sell all your ammo, you should have around 260 military grand bullets in the end. Now you with your 260 bullets buy and sell the cheapest bullets back and forth until the achievement unlocks. Hold down to buy and sell faster.
You will always sell back ammo at twice the price of buying ammo. You can do this with any ammo type, the cheaper the easier this will be to do. If you don't have enough military grade ammo to buy and sell after you have sold all your ammo you may have difficulty pulling this method off.
Once you complete this reload your previous checkpoint and carry on as you would normally.