I have about 400 rounds that I gave away for about what you are having trouble with & every round was winchester slug 2 3/4" 1oz.
Box to box there was to much diff.
Bulls this box not even on paper next box.
That was why I ask obout if you used diff ammo.
Walmart sold some winchester slugs in boxes of 15 that I cut apart and found dif.fslugs,diff charges of powder I belive it all was the same powder
but it might not have been.
Use your 3030 for know, but try some diff.ammo and see if it does the same thing.