This device is designed specifically for the Remington 870 20 gauge with a FACTORY mag tube extension. It will cost effectively allow you a standoff / breaching option as well as an aggressive non-lethal deterrent by simply attaching to your factory tube extension.
This HFD2 Breacher / Standoff was machined to the exact specs. of my Remington magazine extension. Fits like a glove with no wiggle at all, even before I tightened the set screws down.