I've got a mount on an Ithaca M37 (no optical sight at the moment) but it's a pump. Recoil operated shotguns are sensitive to weight gain and may not reliably operate having the extra weight added on. The recoil assembly has to move the extra weight along with the designed weight and thus doing so may not have enough energy left to operate the action of the weapon.
Several companies make mounts for shotguns; Weaver rail, saddle mounts, laser mounts, etc. I even remember seeing somewhere a Pictanney (sp) Rail mounted on a smoothbore.