'Corkscrew' Self-Propelled Slug Thrower
Single shot, breech loaded. 
Round is pulled into rotational barrel and spun upon firing, spinning the round as it leaves the gun.
The round is fired and spun to keep it in the air long enough for it's rocket propulsion to activate.
Once the rocket propulsion system is activated, the round increases speed dramatically.
The round is not explosive, but very heavy. (similar to the coalition revolver cannon).