You might look for some Fiocchi 20 gauge shells. I see them a lot a Sporting Clays range in the trash cans of on the ground for free. You can load those with AA wads and Longshot at 1150, using Fiocchi primers. I like Longshot and have had good results. If you can find two loads they you can see which pattern well and go from there. I also expect there is a Federal 20 load out there to try.