The best way is a hydraulic dent raiser but they are expensive and I have never seen one in 20 ga. That leaves expanding brass mandrels. I got mine from Brownells but they no longer list them. They are not the perfect answer but do work. You expand the mandrel a few thousands at a time and drive it through the bore after each expansion, stopping before you get to the choke section. It takes several trips through but eventually the dent will be raised although it might still have a blemish visible on the exterior.
