I had the same problem when I first tried using a rechargeable in my U2 a couple of years ago. A fellow CPF member also pointed out to me that you have to remove the head to insert rechargeables. You may be surprised as I was to find that an 18650 will fit depending on how old your U2 is. Mine was one of the first and did not have the plastic insert that Surefire started putting in them later. While it can easily be removed this still does not guarantee that an 18650 will fit your particular body. However, Lighthound carries the U2 Leef bodies that will accomodate an 18650.
Well, the 17670 fit without hassle-I wonder if there's a purpose to that insert in the back of the tube...I guess i'll have to rethink how I change batteries now! Thanks for all of your help, fella's! :twothumbs