This is what the manufacturer says in regards to compatibility of the Vltor M1-S: M1A, M14 Improved Stock System. "The M1-S stock will fit most M1A/M14’s with standard profile barrels. Out of the box, the M1-S stock will not fit M1A/M14 rifles with the following features: medium contour barrel, heavy contour barrel, National Match barrel, additional/improved bedded lug, M21 and M25 type rifles. Please keep in mind that this stock system exists in Semi-Auto and Select Fire variations. Please make sure that you select the correct stock for your application at the time of purchase. "