Head to the North Mall across the park. Inside hunt around the shops that are under construction for the nail guns. You shouldn't need more than two. Go back out to the park the way you came in and the cutscene will play. Head to the trees near the Food Court and wait for the jeep to try and make a go for you.

If you stay in the tree area, the jeep should smack repeatedly into the trees, freeing up a major opportunity to nail gun the inmate on the machine gun from a safe distance.

Once he is down, nail gun the passenger, then finally the driver. Once all three are dead you can go examine the .50 Cal machine gun and dismount it. It holds about 200 rds and is extremely powerful (I used it to kill Carlito in one small burst). It is also extremely acurate (you can lightly squeeze the trigger quickly for single shot). One of the best weapons in the game!