As far as shooting qualities, I think Don Amos has recorded moments of inertia for several model 21’s in 20 gauge for the database.
If you can locate that, it might give you a good comparison between the guns that you regularly shoot, and your new gun.
Sort of see where your model generally fits into your spectrum.
If you shoot with any regularity, on a course as you described, I think I would look in a different direction for changes in my performance.
I think I would get a couple of 3 foot square sheets of cardboard, and just fire some rounds at them at the club. It might be possible your stock dimensions or your mounting of the new gun are not sending the pellets where you are looking.
Mostly because a 7/8oz load is more than adequate load at the distances cited, and the chokes listed.
I’m refining an old 410 today, just for those reasons.
It hits too far right, and low for my Mount.
It breaks targets just fine, but I want the pattern exactly where my typical mount places the barrels.
It irks me when I hit straight always left or right of center.
But you just wouldn’t know what’s going on, without shooting a few patterns.
For what it’s worth, furniture stores provide an endless supply of cardboard to shoot at.
