If it isn't marked on the barrel flats when you take the forearm off, then, again IIRC, 26" was IC/M and 28 was M/F in 20 gauge. In 12 I believe it was 28" was IC/M and 30" was M/F, but you are correct, taking a bore gauge to get barrel diameter and choke diameter will get you closer. Of course actual pattern will be determined with your load at the pattern plate
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