Barrel mounted see mine and Illusion's
Rail mounted:
Also here's the gap that needs to be filed to fit over the PTW barrel.(red arrows)
The blue arrow shows the "pin" that locks into the barrel nut and eliminates rotation.
I did find a small piece of electricians tape in the opening and in the front bracket helped to hold everything just a little firmer. It's bombproof now.
sorry for the crappy pic.