Easier: get yourself a Lahti or a Solothurn, get surplus 20mm ammo, turn the drive bands down 0.008 on a lathe, and reload the brass yourself.
Yeah, I read "Unintended Consequences."
Thanks for the suggestion, have seen those weapons. In fact my father was an armorer during WWII and maintained Quad 50's and 20mm Bofors. He has been a hunter all his life. At 84 he is still a very good shot and his gun collection is large. Nicest guy in the world, but don't mess with the old gray eagles.
I had a friend when I live in NM that owned a Naval Cannon which he fired on the 4th of July. Very Very heavy. He had a special carriage built for it, but the steel rims on the wheels were not very wide and it was too heavy to tow on the highway without damaging the road. He had to trailer it to move it. It sat on his front lawn most of the time. (he was the local coroner there).