Yes, I have fired a shotgun outfitted with the knoxx specops.
It does function exactly as advertised. It will extend your range trips if you shoot a lot of slugs and buckshot...however, the springs are STIFF and will hardly function at all with bird shot or other light loads.
Personally, I did not like it.
For one, pistol grips are too cumbersome and bother me (personal preference).
Second, the fact that the entire shotgun will recoil back into the stock makes for a weight experience. Most people tell you they don't notice it...but I did.
I wouldn't own one.
Lots of guys LOVER them over on shotgunworld, though. It seems to be *THE* choice stock on their tactical rigs.
I suggest you try it before you buy it.