The misunderstanding of this was one of the major reasons for changing the marks from "Not For Ball" to "Choke" As I recall most proof laws required one of these marks, depending upon time frame, on any gun having 0.2mm (0.008") of choke or more. It was I believe optional on a gun with any lesser amount of choke. A cylindder bore was of course unmarked.
That is correct under British rules of proof. Guns with a minimum of .008" constriction had to be marked Choke. But the person submitting the gun for proof could request that it be marked Choke if it had as little as .004" constriction.
Last edited by L. Brown; 02/22/19 07:00 PM.