I have no idea about ammunitions but wouldn't the distance and the sheer size of the bullet make it nearly impossible to shoot it accurately at a target? Don't you have to account for wind too?
I think this is a good time to ask, does the "Coriolis effect" factor into a bullet trajectory at that range?
You obviously have to have some skill to do so. Here is some info on the ammunition and this specific gun is at the bottom. The article says you can shoot it accurately up to 1500 yards which is almost a mile. http://www.vincelewis.net/20mm.html
The answer is yes. It takes 3 seconds for a normal rifle bullet to travel 1000 yards. From talking to a military sniper, the marksman has to take everything into account. Wind, earth rotation, air pressure, weather, and gravity.
Every time i hear this music I get reminded of this Ad in Australia. <iframe title="YouTube video player" width="480" height="390" src="http://www.youtube.com/embed/zvfD9-SUeZA" frameborder="0" allowfullscreen></iframe> That said. WOW.