Nordic Components - 12 Gauge Shotgun Magazine Tube Clamp These are the finest machined 12 gauge extension tube clamps currently on the market. This clamp does not move and will hold zero. The clamp is designed to be used on 12 gauge shotguns utilizing a Nordic Components Shotgun Extension tube with an OD of 1.06 inches. This clamp does not require gunsmith fitting and is easily installable by the end user. Please refer to the factory maintenance manual which came with your shotgun for full instructions. If ordering a clamp for the Mossberg 590 and 930SPX shotgun models, or any Mossberg shotgun with heavy walled barrels, do not select this clamp. Select the clamp listed specifically for the Mossberg. No NC clamp will fit the Remington M887. It is also not compatible with original Browning Auto-5 or similar shotguns with reciprocating barrel.