As soon you are able to get a vehicle, find a safe place away from enemy spawn points 
and find a way to keep the right trigger held down (use a couple of rubberbands).just 
walk away for a few hours and after you have unloaded 100,000 shots you will get 20,000 exp. 
note:This only works one time! so dont waste anymore time than needed, i have tested and 
proven this.