Adrain; sounds like you are getting a pretty good pattern with that 20ga. A little thing that I learned really improved patterns is polishing the barrel, never fails to improve patterns at least a little. Also as a general rule the higher temps and higher humidity will provide better patterns so you can expect patterns to decrease a little as it gets cooler. Good luck and have fun, those 20's are a lot of fun!

mr longbeard; I know you did not ask me, but you will have no problem with Hevi-13 #6's to 50 yards and a bit beyond if that is how far you want to shoot. Plenty of knockdown power if you have the pattern to back it up. I use #6's only and generally they shoot about 1020-1100fps.
The farthest I have killed one is 62 yards (12ga Hevi-13 3" #6's).