Many years ago when I worked at Aberdeen Proving Ground there was vertical fire testing where the piece would be elevated to 89-90 degrees (or even past 90 depending on the high altitude winds) to allow recovery of the projectile which would impact a 1/2 mile or so away.
Obviously, the FAA would close off air traffic overhead.
It may be apocryphal, but the story was north bound air traffic would be routed east of APG over Delaware while southbound traffic would be west over PA and central MD. In effect, APG was the Jersey wall between these lanes.
Rule of thumb for a M109A5/6 shooting lo angle:
1000 feet of altitude for every 1 kilometer of range, i.e an 18 km shot goes ~18,000 feet above ground.