If that Carlson choke is nonported I would try the Federal Flight Control lead #6. I would suspect that any lead load you use will not perform as well as those winchester ER heavier-than-lead.

I don't know anything about your barrel and I have never shot an Encore, but the fact that the barrel is only 20" shouldn't limit your 40 yard pattern to that great of a degree. My son now has a Winchester 1300 with a 22" barrel and it shoots a great pattern at 40 yards. Perhaps a more open choke would improve your results - might be worth a try.

Good luck