The legal minimum is 18", but everyone says 18.5 to be safe. You don't want to go to jail for 1/2".
Do a Gunbroker search for Mossberg Persuader. It's an 18.5" barrel, pistol-grip only, 30" overall. Not an NFA weapon.
I've not fired one, but I hear tell it's murder on your wrists.
A stock makes the gun much more controllable and comfortable.