those are an entry-level version of a precision long range sight. they are designed to be used with a front aperture sight. they are for BPCR (black powder cartridge rifles or sometimes SPCR [smokeless cartridge powder rifles]) that are used for long range shooting.
try one of these (not advocating the business just point out the product)
that way you can scope the shotgun for 4 legged critters bigger than a rabbit and remove it for rabbits/birds/etc
here in Ohio we are only allowed to hunt deer with shotguns and I have used a saddle mount on a shotgun before. as far as a scope, all you really need is a
1.75 x 4 x 32. A scope like that will give you a little more light gathering ability at dusk and dawn and will do what you need.
such as this