Panzer Arms M4TBRASSY: This replacement buffer tube/recoil spring assembly is for use when converting a fixed stock, or early model, M4/M4 clone shotgun to accept an adjustable telescopic stock. Explanation - Many of the current production models already have a 5-position buffer tube under the conventional stock so stock conversion on those is easy, all you need is the adjustable skeletonized stock and a pistol grip. However, many of the earlier M4 models had a smooth tube under the stock with no locking notches on the tube. If you have one of these models the adjustable stock will not work without converting the tube, because the stock would just slide right off. This new 5-position tube is made to replace your smooth tube ( if that is what your gun has ), and allow you to do a conversion to an adjustable skeletonized stock if so desired. Telescopic stocks can be purchased separately HERE!