If it mike's .580, it's gotta be something other than a .20 gauge. That would be too extreme for even a super full choke. I'm guessing you have a 24 guage with little or no choke. In a black powder gun I prefer no choke. Read easier to load. Your range is generally 25 yards or less, and all that is needed is to work up a load in whatever shot size you are using. You can compensate for spred with more shot to fill the pattern. Did I mention a heck of a lot easier to load. You just have to work with the gun's and your limitations, and you'll put meat on the table.
Bill
I started with nothing, and still have most of it left!