There isn't one for the SIG 556 yet. We modifiy it to fit. But someone said they talked to the mfg of the grip, and they said they had so much requests they may start making them just for the Sig556.
The CAA UPG for M16/M4 is the one I used. Had to do alot of dremel work, and fitting and more dremel work. Then drill a new screw hole. it's a pain in the ass but the rewards are great. having your grip being able to custon fit is great!