Barrel Magazine Clamp (BMC) - Clamps to the barrel and the extended mag tube. Stabilizes and supports the tube; includes built-in sling swivel stud. Benelli 3-round and Remington 5-round magazine extensions include barrel magazine clamp.
Big Book Catalog, Issue:69, Page:200
SPECS: BMC-R Clamp - Fits Benelli Nova/Super Nova/M4, Rem. 870, and Win. 1200/1300 Def. BMC-U Clamp - Fits Rem. 1100/11-87.