I have an 870 and purchased a SumToy choke. They are a custom choke maker. The nice thing with them is you pay for 1 choke and if doesn't pattern well, they will keep working with you and keep tweeking chokes until the gun patterns well. The owner's name is William Lambert, phone number is 912-237-4360. The website is
http://www.sumtoycustoms.com

I purchased a lot of chokes, tried them, didn't like the pattern and then was stuck with the chokes. SumToy charges you 1 time and you will end up with the choke that works best for your gun.
Out of my 870, I shoot HeviShot 3.5" 2.25oz #7 and get 230 - 250 hits in a 10 inch circle at 40 yards. Pattern is pretty even too.

Good Luck