I need to know what gun I'm missing for this achievement. So far I've fired:

Flamethrower
Pistol
Desert Eagle
SMG
SAW
Assault Rifle
Shotgun
Rocket Launcher

That's 8 guns and I haven't gotten the achievement.

It seems that you have fired all the weapons in the game. Not sure why the achievement has not unlocked for you. About the only thing you can try is firing all the weapons again. Maybe one of the weapons did not register or something. 

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I didn't get the achievement either and I'm sure I fired all the weapons.
Maybe I'll unlock it when I replay this game in the hardest difficulty sometime.

Newb on Tour.

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Without sleep there are no dreams.

 Newb on Tour wrote:

I didn't get the achievement either and I'm sure I fired all the weapons.
Maybe I'll unlock it when I replay this game in the hardest difficulty sometime.

Newb on Tour.

On my second playthrough I got this achievement unlocked. Seems I really did for get to fire a weapon. :)

Newb on Tour.

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Without sleep there are no dreams.