blackwolfe: there are no disadvantages to the microgroove rifling. I have a number of rifles with that rifling and they shoot cast slugs just as well if not better than jacketed slugs. In fact, the only reason I shoot any jacketed slugs down those tubes is to use up the bullets. I prefer hunting with cast slugs.
That being said, you may wish to drop on down to Veral smiths forum, down the line here at Graybeards. In addition, you may wish to visit the Beartooth Bullets website. Veral speaks often to the process of 'firelapping the bore or borelapping' which simply imparts controlled wear to a barrel and allows it to maximize its accuracy potential. Beartooth sells the materials and kits for this process. I think Veral does as well. Beartooth references Veral quite often in its literature. I have firelapped a number of microgroove barrels using the materials Beartooth provides and will now only shoot cast slugs down my tubes. If ya see me shootin' anything with jackets it is just to use them up. HTH. Mikey.