CMOS, I did a test long ago for my agency, using issue 870s with "Police Cylinder" chokes,and my chokeless personal 870, and WW generic 9 pellet 00. At 25 yards,none of 15 rounds kept all the pellets on a lifesized silohuette, but almost all of the misses were one or two pellets. We tried a full choke, and the hit numbers dropped. A Mod choke gave the best results on most shots, but a few patterns had a big hole in the center, a donut pattern as it were. My guess is,a light Modified or Skt II choke may be best for 00.
But testing is the only sure way to know.
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