Available in a kit with Extention Tube & Spring here:
The MXT Extension Tube Coupler can be used to mate two of our MXT Extension Tubes together. When using an extension tube that is longer than the barrel, it can be hard to find a gun case that can accommodate it. The Nordic Components MXT coupler will make it easier for you. It eliminates the need to disassemble your barrel clamp and Barrel nut from the gun when transporting your shotgun to and from the range. This is a very handy item for the 3-gun and tactical shotgun competitor.