rifled barrels are at major diameter at the grooves, and under at the lands. smooth bores obviously only have one diameter through most of the bore unless its one of the modern "back bored" barrels. this means that if you run a button die down the bore of a 20 gauge you are opening up the grooves beyond 20 gauge and thus loosing your gas seal. rifling barrels isnt cheap either. of the few gunsmiths that do it right, expect anywhere from $150 to $300. just pick yourself up a properly rifled, heavier contoured slug barrel and save yourself the headache.