You need an upgraded weapon, I think the place where you can buy an M1A1 Carbine, go to the barrier (not the wall), get on the sandbag on the floor and you see a red box (something that looks like one of the object that activates electric barriers. Shoot it, and a weird ring will form and go up in the sky. They'll say "I want to play a game... Let's play Hide-N-Seek!" You have to find 2 bears and a Monkey Bomb. The first teddy bear is in the room with the Trench Gun. Go near the cages and inside it you'll see a teddy bear with a Juggernaut and a M1911 Colt. Shoot it. Then go outside to the mainframe. Look up the building at the left, in the windows the teddy bear is looking down at you. Shoot it. Now for the Monkey Bomb, go in the room with the Thompson and fireplace. Go to the fireplace, it may be a bit hard to see, but inside it you see the Monkey Bomb. Shoot it.

Game Over.