This device allows you to use the rifle and optic to dryfire practice in a very small area.  With this adapter most scopes focus in the 11-15 feet range, depth of field is about 6‚Äù.  By increasing your parallax distance you can sometimes increase that focal distance.  With proper lighting you can use your scope at maximum magnification.  Perfect for in the house, in the garage or a small backyard.  Use this device to practice different positions, balance points on the rifle, slung vs unslung, various support bag combinations. I have found the D.F.A.T to be extremely valuable in determining how small I can make my "wobble" with non-prone positions.  Usually shooters can get a good idea of their abilities and improvements by dry firing on known "MOA" size targets. 

The Gen 2 version of the DFAT has the aperture lens system held within a threaded aluminum sunshade. This is a very precise fit. Be sure to take care when fitting, as you do not want to cross thread your scope or the adapter.

The 37 DFAT HD Gen II has the aperture lens system held within a threaded aluminum sunshade, like other Gen 2 DFATs. However it's significant benefit is the glass lens is held in with a threaded retainer. It is possible to remove for cleaning if needed.  This design provides a very secure fitment like lenses in a scope.    

Another advantage of the 37mm Gen 2, is the smaller diameter allows one DFAT to fit more scopes without any barrel clearance issues.