Charlie;
The reduction in bore diameter is what gives this gun a full choke. A 16ga. bore is .662, an 18ga. is .637. The difference is .025 which is a fullchoke in a 20ga. The same principle applies to any gauge. Take the 12 ga. for example. Nominal bore dia.= .729, a constriction of .036 would give you a diameter for the choke of .693 which is the diameter for a 14ga. The Wm. Ford 20ga. chokes are parallel for the entire length of the choke (3.75").
Hope this helps.
Daryl
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