Your M1 S 90 barrel will fit on a M2 but the reverse isn't true. The M2 barrel has a longer lug that attaches the magazine ring on the bottom of the barrel than the M1 barrel. The M2 barrel has a ring at the rear of the barrel that the M1 does not have; You have to modify the M1 foreend to allow a M2 barrel to be used with a M1 or purchase a M2 foreend.
