I was patterning a 20ga 870 compact gun yesterday with 3" 1.5oz Fed. HW #7's and a Sum toy 562.5 and a Trulock Fed. HW .560 choke, first shot was really nice pattern 167 in a 10" at 40yds, the next 3 shots completely blew the center of the pattern out . Trulock choke did 150 but same thing . you could plainly see all the pellets around the center with a large hole in the middle. For the record I was shooting at a 36x36 piece of paper "new" for each shot, I dry brushed and run a dry patch down the barrel between each shot.
So my question is what do you think the problem is?
P,S I did deep clean the barrel as per Clark Bush'es guide lines when i got it. I will say this the tooling in the barrel is terrible it looks like a washboard dirt road.
I would try shooting it 3 times without brushing in between. It doesn't make sense why the center would blow out, but some guns shoot better with a dirty barrel.