in phil cassidies army store (yard) there are two ways to enter the area were the tank is. through the gate, and around the building in the, sort of, ally thing. anyway were the ally meets the tank-yard you have the corner of the building, and the wall about two meters to the side (thus forming the ally). get a car in there so that the back of it is against the corner of the building, and turn your wheels to face the wall of the yard. reverse against the corner of the building and slowly the front of the car will creep closer to the wall of the yard. wen the car toushes the wall it will be catapaulted into the air. iv only just found this. so iv only done it a handful of times. works well in the police car. terrible in the idaho. and my taxi teleported to the inside of the building. wen i drove a little further in jumped into the roof, and caught fire, then wen i got out i appeared on top of the roof.
this mite be a completely useless glitch, unless your lucky enough to ger on the roof, then mabe it could help with the arms shortage mission. but its worth cheching out. mabe its not isolated to this particular sight.