I'm thinking, a piece that is on wheels pulled behind a truck, capable of a 20 mile range, with explosive rounds.
Well explosive rounds are another problem - iirc there is a $200 destructive device 'tax' for everything that goes 'boom' outside a barrel.
Not sure if there is anything that outright bans modern artillery, but they would be over the .50 caliber limit and not a shotgun - therefore subject to more regulations.
Been a long time since I;ve read about cannon ownership but I believe old style cannons are allowed due to them being black powder devices.