This achievement/trophy is incredibly simple to get in Metro 2033, on easy difficulty with the Ranger Pack installed. At Armory station, you are presented with a free Volt Driver and a free Heavy Automatic Shotgun, both of which can be traded at the gun table for a Duplet, which nets a proft of at least 200 MGR per transaction. Thus, if you have saved about 150 MGR up until that point, you are basically guaranteed to get it. Then one can simply reload the checkpoint and keep the gun of your choosing if you wish to keep one.
However, in Metro 2033 Redux, you no longer get these weapons free, so more trading must be done to achieve the required 1000 rounds - a good tactic is to simply sell all of your weapons near the end of the game, get the achievement/tropy, then reload the checkpoint to retrieve them.