The Orange Box

The only bullet you have to use is a lock when you crawl out of a vent.

Other than that, using grenades, rockets, the crowbar, and the grav gun are the only things you can kill with.

The grav gun is great for quickly grabbing something and chucking it at an enemy.

You can play on easy if you need to. Other than that, this is a really easy one. You can do it in one sitting.

The one bullet must be fired during the level "Lowlife". The room where you get the shotgun and the 9mm is entered through the A/C vents and can only be exited by SHOOTING the lock on the door. If you miss the lock, then you should stop playing video games.

After you fire your one shot, proceed through the game whilst only using grenades, rockets, crowbars, and the Gravity gun.

Grenades are limited so use against large groups of enemies and snipers located in the windows.

Rockets are mainly useful against gunships but can also be used during the civilian evacuation near the end as there are plenty available in both scenarios.

Apart from using the crowbar in confined spaces, your best bet is the Gravity Gun since it essentially has unlimited ammo and kills most enemies in one hit from anything.

If you do feel the need to cheat (even though this achievement is already very easy without them), use the above Invincibility cheat to get in close with the crowbar and use the Ammo Spawn cheat (Y, B, A, X, RB, Y, X, A, B, RB) to restore your rockets and grenades.

This is the easiest way of achieving this. First, it doesn't matter what difficulty you do it on, because we will be uning cheats. (Copied from cheatcc.com)

Invincibility:
While playing the game, press LB, Up, RB, Up, LB(2), Up, RB(2), Up.

Enter the code as shown. Do not pause the game, just enter it as your playing the game. Codes do not disable anything, including saving and achievements. I believe you have to type in the codes every mission.

But complete the game by only firing one bullet. When you get to the part with the lock in the beginning, that's when you shoot. ONLY shoot there! If you shoot again on accident, load your previous save game. You CAN use grenades, crowbar, rockets, and the gravity gun.

The only thing i would like to add to what has already been said above is that you MUST PLAY FROM THE BEGINNING OF THE GAME.

you won't get a gun till chapter three, but you still must play through the first two chapters.

i tried skipping to chapter three, i beat the game with only one bullet, and it didn't pop. i had to start all over.

I would say, all of the other contributions; with the exception perhaps of the cheat code as I don't know whether that's valid, are great answers. My contribution to this, is a very quick video showing you where you fire that one bullet!