Present day chokes generally have less constriction for a given "Choke" than did older guns. There was never an "Official" requirement for a given amount of constriction to mark a bbl with any choke marking.
However in the old card & felt/fiber wad days generally constrictions were on the order of;
Full = .040"
Imp Mod/3/4 choke = .030"
Mod/˝ choke = .020"
Ľ choke = .010"
Imp Cyl = .005"
Cyl = 0.000-.003"
It is thus noted this .005" increment applied only to the Imp Cyl, the majoor chokes going up in increments of 010" per choke. This is all for the 12ga of course. In metrics the full was about 1MM, with others in proportion.
Other gauges would, from a knowledgable maker, have a similar constriction proportinal to the bore as explained by R'Man.
