The product that all of you guys are linking are the Knight's Armament Standalone buttstock module. It's not the same as the Daniel Defense interpretation. It's like suggesting a G36 when someone's asking for an M4, just because they're both carbines.
http://www.knightarmco.com/m203_12ga.html
In any case, I have one of these and love it to death, although I'd originally had it seized by customs.