RE: mossberg slug barrel
My brothers 500S I believe it is, anyway, a Mossberg 500 model slugster. His is drilled and tapped to carry a scope. Since you mentioned a drive I would probably use open sights since you never know how close the deer will pass you. I don't know the situation where you hunt, but where I hunt I have had deer run just feet past me and I would prefer the open sight for a shot then.
Good luck