Have been corresponding with the company trying to figure out what I have that will fit the bill (pun intended). The Mossberg extended mag tube is apparantly out as they need 1 3/16 inch diameter and approximately 1 1/2 inches of barrel to mount the duckbill and the Mossberg barrel lug is approximately 1/2 inch from the end of the barrel. My Benelli and Ithaca have rifle-style sights on them. Thinking either another one of my Mossbergs (one of the 5 shots) or buying another Ithaca barrel. Thinking of going with the Ithaca as it has no disconnector
Will decide after hearing back again from them.