RE: slug hunting
Scopes also don't "gather" light. They magnify the image which is why you can see it better. Use a 1X scope and it will not give any light advantage over the naked eye, if anything it will have less light. Scopes always absorb light, some just less than others but they never increase it.
And like said above the biggest difference in a shotgun scope is the parallax setting. A rifle scope will be between 100-150 and the shotgun version will be 50-75. Some manufacturers will adjust it for you if you want though. I am pretty sure Leupold will.
Again I wouldn't put a scope on a shotgun receiver, even if it's drilled and tapped for one. Unless you plan on pinning the barrel. Your better off with good rifle sights.
Paul