ML Shotgun for under $400? Tough call for a traditional and even an inline shotgun will be between 300-400.

I would also suggest a T/C New Englander. If youcan find shotgun version great or you can typically pick the rifle up used for 100-150 in nice shape and the ala-carte the barrel from fox ridge outfitters (T/C Custom shop). They sell the IC barrel for $175. Or you can surf Ebay for a NE shotgun barrel or perhapsthe lock and stock then buy the barrel.

http://www.foxridgeoutfitters.com/index.cfm

or,
CVA now has a Optima shotgun version, or you could snag up a used Optima rifle and ala-carte the shotgun barrel.
www.cva.com

The Knight TK2000 and White Tominator are excellent inline shotguns. But you will need to search for used as new they will be over $400.



Good luck!