I'm not sure what the proper name for it is, but there is a simple tool for coaching a shooter that's just a 45-degree angled piece of two-way mirror that attaches behind the rear sight. It allows a coach off to the side to see the same sight picture.

Here are some pictures if my description isn't clear. I'm intentionally not embedding these pictures, as they belong to another forum.

http://www.usmilitariaforum.com/uploads//monthly_05_2012/post-92-1336724212.jpg

http://www.usmilitariaforum.com/uploads//monthly_05_2012/post-92-1336724161.jpg

Does anyone know if there is an AR15 version of this tool? At the very least, is there a common name for it?

We used to call them Belgium Sights when I was an instructor at PISC. I tried finding one a few years back with no luck. Would love to know where to get one though.