Ballistic Products has some pretty good articles on folks who have worked with different materials to convey the payload out of the barrel with good dispersal. I have not patterned my 20 gauge shells yet, but on the 12 gauge buckshot loads I use a gas seal and wrap the buckshot in a Teflon wrap with buffer media. The pattern is about 10 inches diameter at 25 feet with OO 9 pellet.