I seen a picture of an AR with a shotgun mounted underneath the barrel, I think it was an 870 remington. For some reason would that be illegal for a civilian to do that to their ar ; and if their is no law against it does any one know were I could get the mount.  P.S. if any one has some pics I'd appreciate if we could see them .

What you saw is called the "Masterkey" and is manufactured by a couple companies. I can't remember exactly which ones, but they are out there for about 450 or more. As for legality, yes they are legal, and you have to register it as a AOW (any other weapon), thus your state would have to allow NFA weapons for it to be legal. I believe they hold about 3 rounds of 2.75 inch 12 gauge shells. Pretty neato if u ask me.

Knights Armament Company is a manufacturer of the 'masterkey' shotgun for the AR.  Named the 'masterkey' because it will open any door.  Check out the link below for more info and some pics.
www.ghostrecon.net/html/arms_masterkey.htm

A couple notes on the MAsteryKey:

First off they cost about $2000.  I've seen them for slightly less, but count on paying about $2000 for the gun and the mount.  The MasterKey was not intended to be used as a sawed-off shotgun.  It was primarily developed to be used with lock buster rounds to breach doors.  Hence, the name MasterKey, as it will open just about any door.  Also consider that the shotgun will become your foregrip.  All points aside, it sure is neat!

from a legallity standpoint, i think you would have to register teh shotgun as an AOW (if legal in yuour state). you may be able to find somone who could work up a system for less than the knight system.  for extra cool points you could go for a auto shotgun.

$999

@www.mr40mm.com

but remember that thing is goign to make your gun real heavy!