The thickness of the mag catch tab on the mag is critical. The statement that it was 'thicker' jumped out at me. The mag catch is designed and installed to work with steel tabs, and there is a little play in most rifles, at least with an empty mag. When loaded, the pressure of the top round pushing against the bolt tightens up the fit. With a thicker polymer tab, you may have trouble getting the mag catch to snap over it. If you ever relieve a rear tab, be sure to work only on the lower side where the mag catch rides and be sure to keep the same angle. The top flat sets the height of the feed lips in the gun and usually shouldn't be modified. In these cases the question comes up whether to modify the mag catch or the magazine - I go with the magazine. If you filed on the catch, it would be too loose for other kinds of mags.