This achievement can be a pain if your don't adapt you play style to it. The original game only required 500 MGR rounds which was much easier to obtain, not that this achievement is much harder.
Military-grade rounds or MGR are the currency used in the Metro. You can shoot it away with any machine gun, but it is advised to hold onto it and spend it on valuable weapons.
I would advise these following tips which makes sure you will unlock this achievement.
- Never use your MGR as ammo. You can hold
/ TAB to check if you are not using your MGR when using a machine gun. If you shoot an enemy and it lights on fire then you are using you MGR so watch out.
- Explore the game! There is lots of ammo hidden in corners and around rooms. Playing slow and exploring the game nets you the most stuff.
- Sell all but one, preferably you least valuable, weapons to the vendors in the metro. This will give you a small boost of about 100/200 extra rounds each time you come to a market. You will frequently find new guns scattered across the metro, but since you can only carry three at the time sell them off. You will find new guns in no time anyway.
- Similar to the above tip, sell your ammo at exchange kiosks. I always sell all my ammo except for 1/2 clips of the weapon I own. There is abundant ammo to be found so you will very likely have no issue selling everything.
If you stick to all tips and explore the world vividly you can unlock the achievement as early as in the DEATH CITY.
Levels that have vendors are: EXHIBITION, RIGA, MARKET, ARMORY, DEFENSE, POLIS.
After obtaining this achievement I would advise you to also do this one: