I have never used one personally but from what I have been told is you had better have your scope mounted very foward on the rifle or not have a scope at all because of the way the stock works.

Rather than your shoulder absorbing the recoil and your body moving rearward with the rifle the stock now absorbes the recoil so your body stays more stationary and the stock continues to move toward your head and likely making contact.

I would be more apt to use one on a shotgun than a heavy recoiling scoped rifle.