Metro 2033

There are three places with the machine gun, the first is Cursed, the second is Front Line and the Third is Trolley.

The kills accumulate as you progress the story, think of it that it saves your kills when the game saves, so if you load a chapter, like, Front Line, the kills you had in cursed are saved but not the ones you did in Front Line or Trolley.

In Cursed, you can use the machine gun to pick a spot where the mutants will attack, the mutants will not stop spawning until you blow up their point of entry but the problem here is two fold, the first, the machine gun only has 50 bullets, the second is that the other NPCs might steal your kills. I personally unlocked it here, took me around 20 minutes, you have to be patient and just aim at a single spot and quickly shoot when a mutant appears. The spot I picked was the left archway entrance from the left tunnel, close to where Khan is. I maybe killed 1 or 2 mutants every 1 or 2 minutes but I got it here.

If you run out of bullets in Cursed and still have not unlocked it, just proceed the game and get to Front Line.

In Front line, sneak into the machine gun on the other side of the bridge and use it to kill the Reds that are on the beginning of the stage.

You can kill five or six enemies this way.

If you still have not unlocked it, go ahead to Trolley where you man a machine gun on a railcar, here you can kill 8 or 10 more enemies if you are good and quick, it should unlock.

I wish you all best of luck!

Adventurers can retire until age 57, you can see their age at their character screen.

To retire them, click on the retire button once they are old enough.

Remember that you can fast forward time if this gets hard to get since time does not pass inside the Endless Tower.