Chapuis makes a 12 ga. double that has rifle sights and is regulated with brenneke slugs. I have seen some test targets and they looked good.
I have had very good results with my 18" 20ga. double and my Browning 12ga. Citori O/U shooting slugs to about 40 yards. Just go to a well stocked gunstore buy 10 of every kind of slug they have and do a test. Start at 25 yards and work your way out to as far as they still shot good. Then hunt with the best grouping brand. If your double will not shoot slugs very good at all, just get a Remington 870 slug gun and go find some pigs.