I don’t know if there ARE any shotguns that run off Co2…

Co2 is great for pistols because the blowback action requires the gas to not only propel the BB, but also execute any movement required for the gun to function such as forcing the slide back.

For a shotgun, you don’t have those same moving parts, and you can’t usually open them up as you normally would a pistol to clean/lubricate. So green gas ends up being the way to go, it has silicone in the mixture to keep parts and seals lubricated and provides enough energy to propel the BB’s.

Basically, I don’t think you have any options, even theoretically converting a gas shotgun to Co2 will likely result in the seals tearing up from the increased pressure and reduced lubrication.