Nordic Components MXT Magazine Extension Nut, Combines with MXT Tubes to Form Complete Extension Kit, Compatible with Mossberg 590, 830, 835, 930, 935, Not Compatible with Mossberg 500 NUT-MB-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.