Get killed so that you end up outside the hospital, then go out onto the road and steal a car. Position the car so you can skip over the demo barrier. Once on the other side, follow the barrier down the side of the hospital until you reach a wall over the road. Once at the wall, follow it all the way down to the right until you come across some yellow boards. Jump them and continue up the little alley. Turn left at the end of the alley then right onto the sidewalk. Follow the sidewalk all the way up until you see a basketball court on the upper left of the screen. Go over to the basketball court and jump the fence or walk through the gap. You can now pick up the AK-47. It constantly respawns, but you can only carry 30/500 rounds.