I'm looking for a magazine tube extension for my Benelli M2. On DMW website it says a 10+ extension is 16" / 9+ is 13" / and 8+ is 11". Does this sound right to anyone? sounds awful small to me. My M2 is a 28" barrel (12 gauge). Other Manufacturers seem to put a 7+ right around flush with my 28" barrel which would put their extensions appx. 17.5" in length. I'm confused. I was originally looking at Nordic, but Benelli is no longer in their drop down list. And Midway says they're discontinued. Any help in this department would be greatly appreciated.