I had some of the Colt AR6721's come in with the 6920 lowers on them. They show the M4 Carbine and 5.56 markings.
You can order one here:
http://www.jlowesguns.com/Colt-AR6721-Tactical-Carbine-HBAR-MD-Compliant-_p_60.html
You can order one here:
http://www.jlowesguns.com/Colt-AR6721-Tactical-Carbine-HBAR-MD-Compliant-_p_60.html