Nordic Components MXT Magazine Extension Nut, Combines with MXT Tubes to Form Complete Extension Kit, Compatible with Beretta 1301, Xtrema, Xtrema2, A400 (Extreme Only), CZ 712/912, Weatherby SA-08 NUT-BR-12-00 The MXT extension nut allows for installation of a Nordic Components MXT 12ga shotgun extension tube kit. The nut replaces the factory magazine cap and is required for installation of the extension tube kit.