If it's not clear from the pic, the gun is in a crate sitting on a truck outside.  After you enter the main gate of AIP, take a left at the first turn (you'll see a barricade and lots of soldiers), and then an immediate right once you're past the building.  Crate 667 is almost all the way in the back, on the right, just before the second entrance.

You're in for quite the battle if you stop to fight.  You may find it easier just to kill the guys closest to the crate, grab the gun, and escape.

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