Choke is "constriction," as in an internal converging cone (IMO, so that the shot can make an orderly journey as it passes through, dropping shot column pressure for some additional velocity).
Smaller gauges require less constriction for the same choke effect; the area of the bore as compared to the area of the constriction throat is the issue.
Metric bore and choke are given in milimeters. A bore of 18.4 mm X 0.03939 in/mm = 0.725" bore. A choke of 18.3 mm is 0.721"; 0.004" constriction = skeet or so.
Hope that helps. Post back if unclear.
