Remington 870 Express - 18" barrel - 12ga - 3"chamber - 6+1 rounds with mag extension - Many collapsible stocks available - many after market attachments and parts. About $315. Also, several easily enterchangeable barrels makes it multipurpose.
Here is the link: http://www.remington.com/product-families/firearms/shotgun-families/pump-action-model-870.aspx
Here is the link: http://www.remington.com/product-families/firearms/shotgun-families/pump-action-model-870.aspx