Couple of my favorite 20s for grouse run .003/.010 (M-21) and .007/.017 (AS20EL). .003 is as open as I want. .010, a first bbl LM, can be made to work using soft shot, promo loads, etc. Tighter than that starts to wreck birds. We can hunt grouse till the end of December here, so a fairly tight 2nd bbl isn't a horror.
I would do as Mike advises, and try spreaders for a season. And I agree with Buzz: I have no use for true cylinder subgauges (suppose Chuck's forcing cone treatment might change my mind.
Sam
