Floating Machin Gun!
All you have to do is, go to a mounted machine gun and get on it but make sure some ones on it first. Click X to get on it. Now you can't get off of it. Pause the game and go to redeploy, and go back to the gun. In the middle of the street their should be a gun floating. Drive a car under it and it will be on top of the car. Get back on it. Then get back in the car drive out from under it, and get back on it. You should be able to drive the gun! Have fun with it.