*This is a Metro 2033 achievement*
This will be easier on Spartan mode because you will have a lot more ammo and will not need to buy anything. I only bought a couple of weapons through the game and saved most of my buying/money achievements for Polis. Be sure to loot everything and if you find a safe, look for the key close by. I got 1000 in the mission "Front Line". The safe at the end of the mission gave me 150 military grade bullets so it really helps to stop and look for the key if you find a safe laying around.
If you go for an achievement like the trade 500 bullets achievement, reload the checkpoint when you are done to keep your military grade ammo.
**Note** This achievement is getting reports of being buggy and not unlocking.
The reason for this is currently unknown.