Jack,
When you see choke listed in thousandths of an inch as you described (.004&.018) that is the amount of actual constriction between the bore diameter and the muzzle diameter. The reason those cylindrical choke gages are not always accurate is that they assume the gun has the standard bore. Many of these old doubles have been honed out to remove pitting or in some cases to increase restriction and in effect increase choke.
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