I have used the Extreme spreaders from BP for loads in my 16ga guns. They work great and are not very expensive. BP even sells a brush wad for your 20ga that when combined with a spreader does open up your patterns well for close range shots. For my 16ga I cut the shot petals off of regular SG16 wad{as recommended by BP) and the loads shoot great without having to reduce the amount of shot.

Byron