Worse pattern after a deep clean?!?

[Gutsdozer]

Ok, I am probably the only person this has happened to. I decided to deep clean and polish my 870 express. I was getting pretty good patterns before the deep clean. I was averaging 230-240 in a 10" circle at 40 yards with 3" Hevi-13 #7 shot. So I thought it could only get better with a deep clean right? Not so much. The number of hits is averaging around 200'ish which I can live with now that the season is here. However, the worse part is that there are now gaps in my pattern. Mainly I seem to be shooting a doughnut hole now. At 40 yards there is probably a 2"-3" diameter hole consistently in my patterns. I tried my LB 3" #5 and it is still there. Also tried the 3" Mag Blends and it isn't quite as bad but it is still there.

What in the world is happening!?! Have I gone crazy??? Should I shoot a few shells threw it and leave it a bit dirty then pattern it again?   

[Longshanks]

Go shoot a couple of boxes of any kind of shell then start shooting the turkey loads. Going from 240 to that doesn't sound good but it may shoot better after a number of shots. If not take it to a gunsmith and let him look at the gun. Gunsmiths have polishing tools and should be able to correct the barrel as long as too much material wasn't taken out of the barrel. Worst case scenario you can buy a 21" barrel for that gun new for $200.

[THattaway]

You mentioned polish. If you used the scotch brite method you probably changed the bore diameter slightly. Now your bore to choke ratio has changed and your pattern has changed. You didn't mention if you tried a different choke. Might try one slightly more open. Usually the polishing tightens things up for most folks, might have pushed your set up to overchoked. Just some thoughts.

[allaboutshooting]

When you deep clean a bore, you do not use any abrasive substance of any kind, only a good solvent, like Hoppe's and a good bronze or brass brush to agitate and help remove the gunk that can be in the bore. It's the combination of the mild chemical action over a few minutes and the agitation of the brush that cleans the bore.

It's really the only way to determine what your barrel "likes". Some barrels like to be squeaky clean, some slightly fouled and some dirty. By deep cleaning and then shooting one round, followed by another, without cleaning between shots, you'll find out just what your barrel likes.

Thanks,
Clark

[WisTurk]

I know my Mossberg prefers a slightly dirty barrel, but not filthy.  Running a bore snake after getting the barrel good and dirty seems to be the sweet spot on mine.  Good luck with yours.

[Longshanks]

You mentioned polishing the barrel. You might want to describe what that process was and Clark can tell you how to proceed from there. My 870SM was polished at the gunsmiths and quite a bit of material was taken out because of the roughness of the barrel. Deep cleaned and shot a few turkey loads out of it and it was terrible. Deep cleaned the gun again and went to the range and shot 45-50 shots and then cleaned it out with a tico tool. Then the results came.

160's in a 10 @ 40. Winchester XX 3/2/6's
280's in a 10 @ 40. Hevi 13 3/2/7's