If you are shooting sabot slugs then a rifled slug barrel will be needed andthe chamber on the slug barrel will only be 3" long and the bore diameter will be of proper size for the slug to engage the rifling.

Some smooth bore barrels of 3.5" guns(Mossberg 835)are over bored so you can't shoot rifled slugs out of them due to the fact that the bore diameter of the barrel is larger than the slug itself.

As far as shooting actual shot shells you can use anything from 2.75 - 3.5 inch shells with no problesm what so ever.