26" 870 choke and load combo?

[gophert]

Yep, barrel is polished. I shoot on calm days with low humidity and temps in the 60's/70's. I shoot at an approx. 36"x36" piece of paper (lasered at 40 yds) with a dot for poa. I have a 10" dia. circle of clear plastic and move it over the densest part of the pattern before drawing my circle/counting. I also run a bore snake in between shots.

Yep..this^^^^^^^^^

[Sand Man]

Exactly like I do other than I hand roll my own loads.  I do take a wire brush and scrub my barrel after every shot then run a bore snake when I'm patterning, but I can't imagine it makes that much difference.  The only other thing I can think of that would cause the drop in pattern would be variability in the ammo, but I can't imagine factory loads varying enough between loads for you to ever see it when patterning.  I'm stumped.

Are these loads you are using to pattern from the same lot number?