Double check to make sure your choke is tight. I learned that the hard way. I had my 870 all patterned a couple years ago and when I checked it before the season the pattern was horrible. Turns out the tube was slightly loose. It patterned well again after I tightened it. I have a differrent 870 now and use Federal Mag-Shok FC 3", 2 oz., #6s with a Jelly Head .665.
7 pellets in the vitals
81 pellets in the head & neck
196 pellets on a 12"X12" target