When you measure a barrel for choke, you are measuring constriction, not choke. Constriction has an effect on choke (the main one) which is defined as the percentage of the shot load striking inside a circle of defined diameter at a defined distance. We hope the constriction reflects choke, because it is easier to determine by measuring the barrel itself, whereas determining choke requires patterning, which requires counting the shot in the load and counting the number of shot pellets striking the board within the circle. With this information, a percentage can be determined. In addition to constriction, the shot size, shot load, charge weight, bore size, protective devices (shot cups, etc) can affect choke. If a premium shotgun is made to order, the customer will typically be asked what choke is desired, with what specific load, and the constriction will be adjusted to give a pattern percentage within the range of that choke. The choke will then be marked. If an economical gun has the choke marked by constriction alone, it may or may not pattern to that percentage. It would be bad to have a "falling out" with a friend or lose a customer over a question choke vs. constriction.
Mike
