Find an old B-80 -- it's basically a Beretta 302 or 303 (I forget which) but either way, it's a good gun and you can probably find it in your price range.

unless you can find a used browning that someone doesnt know what they have,i dont think you will find it for 400 or less.there are good auto shotguns out there but new you will pay double that price and higher.bass pro carries an express auto for around 200,it is a 12ga auto,plan black,i have heard that is an ok shotgun.i hope this helps.