Replica muzzle loading guns made to various scales are legal
They have to be muzzle loading to be legal - no cheating, but they can be electrically detonated. A bit harder to hide in the bushes, but a replica or two smaller than full scale could probably be set to sweep the approaches to good effect. Scaling down and using tungsten shot would probably keep the effectiveness of a larger gun firing lead shot - just a thought. And muzzle loading isn't much of a disadvantage in the extreme situation of a hit squad coming to confiscate your arms and kill you, since it's not like you'd live to reload them anyway
Comment about the battle of Trafalgar
The first gun fired by Nelson's Victory was a carronade. HMS Victory used the two 68-pounder carronades which she carried on her forecastle to great effect at the Battle of Trafalgar, clearing the gun deck of the Bucentaure by firing a round shot and a keg of 500 musket balls through the Bucentaure's stern windows.