look at the side wall of the choke tube. The thicker side wall is the tighter pattern. I don't know what numbers line up with what choke, but they are skeet, improved cylinder, modified, and full. The full is the tightest pattern.
Go to the gun manufacturer's website to find out what you need. They will take great pains to insure you do not hurt yourself, someone else, or that you will get the best performance from their product.
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