assume your double is a 12 bore, correct. I'm wondering if there isn't enough "gription" at the shoulder pocket with that checkered butt surface. Do you have a padded vest, hunting jacket or other wearing apparel that thenface of the checkered butt on your shotgun can adhear to? If it were my gun, which of course it isn't, I would try this "trick' on do-it-yourself gun fitting-- cut a piece of a medium grit sandpaper to the outline of the butt on your shotgun, put it with the rough side out, vis- the plain side stuck (temporarily) to the butt checkered surface with a doubled loop of masking tape, once it is secure, try the mount, and even shoot to see if your gun still slips. maybe all you may need to solve this problem is to have the surface of the checkered walnut of your gunstock "pointed up"?? RWTF