Are both guns slipping? The pitch on your stock is wrong for you if you have to reposition it after each shot. We could get into a long discussion about pitch but you might want to try a few simple things that might work for you.

Wear a shooting shirt or vest with a rough leather/suede surface patch on your chest.
Concentrate on controlling the grip firmly pulling it snugly into the pocket of your chest when you mount the gun using your forward hand only to support and guide the barrels.
Buy some RTV/clear 100% silicone and apply a thin coat to the butt wetting your finger then smearing it to cover the surface. This will peel off easily if you need to but will give you a non slip surface.
I generally don't like recoil pads on shotguns unless they were ordered from the factory like that as they will add weight to the stock and disturb the balance of the gun.

Good luck and let us know how you make out.

DHBP vs rubber
 

Thanks all for all your advice going to look into the rubber coating and the Beretta gel pack :smiley7::smiley7: