The bullpup concept has been around since the early part of the 20th century and involves a system of shortening the overall length of a long gun by moving the action back behind the trigger. With shotguns, there are two modern day examples in the Kel-Tec KSG and the UTAS UTS-15, both of which retail for far more than your standard 870.
Read the full article at Shooting Illustrated. http://www.shootingillustrated.com/index.php/34903/bullpup-unlimited-remington-870-shotgun-conversion-kit/
You can also watch the video shooting test here.