For Trap Doubles - and for Trap Singles from the 16 - 20 yard line I use a Modified Choke.
From 21 - 24 yard line I go to an Improved Modified.
From 25 - 27 yard line, I go to a Full choke.
I don't change my shells - for Trap singles, Doubles or even Continental Trap ...( not even at the 27 yard line...where I shoot for fun, not because I've earned the handicap distance ...) but I stay with 1 oz of 8's at 1225 fps ....( and I use the same shell for sporting clays in 12 ga.)