hi......i recently picked up another Mossberg 500 with a mag tube/barrel lug i have not seen before. The mag tube itself is threaded outside, the "ring" on the barrel lug fits OVER the mag tube instead of being the same OD as the tube and the barrel nut threads on like a cap as opposed to the barrel nut having a threaded stud and threading into the cap on the mag tube. the end of the mag tube is open,if you shine a light in the tube you can see the spring and follower retained by some kind of clip inside the end of the tube. Never seen this setup before....will try to post some pics and would welcome some input....