This device designed specifically for the Mossberg 500 12ga. It simply attaches to the Take Down Screw giving you a cost effective, breaching / standoff device.
Your Take Down Screw needs to be the newer version, 1" in diameter x 3/8" thick with a threaded swivel hole in the middle.
This device is designed to fit a Mossberg 500 with a staggered length barrel to mag tube ratio, such as commonly found on Mossberg 500's with an 18.5" barrel.
Machined from 1144 Stressproof steel and plated black and may exhibit some signature marks or imperfections. 
It will come with all assembly hardware, including a 1.0 ml package of VC3 Threadmate, and installation instructions. 
All you'll need is a phillips screwdriver. 
Dimensions are as follows:
An OAL of 4.627". A working length of 4.22" from the face of the take down screw to the tips of the teeth, and an OD of 1.250. Dimensions are approximate.