Why stop there? Why not a 105 mm deck gun?

You can buy a nice M2 (for about the price of a 25' center console, plus a $200
tax stamp) but you can't own anything with a bore greater than .5" unless it is
a shotgun or muzzle loader (Gun Control Act of 1968).
You could get a few muzzle loading cannon though ;-)
"Arrr, come along side and lay a broadside among her timbers matey!"