I agree, don't waste your money on saboted slugs for your smooth bore. They are expensive and most likely will not shoot very well any way.

My mossberg 500 is good for about 60-70 yards with rifled slugs. Some combinations are better, some are worse. Get a bunch and see what happens.

I don't know that I would waste my time with a scope on that set up. Especially one mounted to the reciever. It may not hold it's zero very well and surely will not if you remove the barrel to clean it. Not to mention I doubt the combination will be accurate enough to warrant it. See if you your beeds line up, or get some of the clamp on fiber optic sights for your vent rib. They work pretty well. Not that a scope would hurt, I just don't think you would need it in that set up.

If you want better accuracy spend 150 bucks on a fully rifled cantilever barrel for your mossberg. Some smooth bores shoot a slug pretty well though, you will just have to see.

Paul