 Hence, 40 points of constriction would correspond to a constriction of 0.040 inch (≈ 1 mm) in the inner diameter of a choke, corresponding to "Extra Full".The marking is usually stamped on the underside of the barrel for older guns without choke tubes, or is spelled out in abbreviated text on the barrel near the gauge marking.In the case of choke tubes, the amount of choke for each barrel is usually stamped on the side of the choke tube, or there may be thin slots cut in the exposed rim of the tube at the barrel opening with the number of slots corresponding to the number of *'s in this table.