Double stamped, it can not be both! 20ga. bore is normally .615 , IC = .007 IM = .020 Most modern guns have about 2-3" of choke, older guns from 1930-s and earlier may have 4-5" of choke from the muzzle. So if the bore is 615 and the shot exits the muzzle at .622 that give you .007 of choke. The only way to get true measure of choke is with a inside bore reader. You must know the inside bore diameter ahead of the choke. That is why a caliper or wedge type tool is misleading. SXS ohio (-:
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