The exact amount of constriction has always varied some by maker. Many makers however followed the system of .040" or 1mm being full choke. 1/4, 1/2 & 3/4 were then fractions of that number IE .010", .020" & .030" respectfully. I/C was with these makers around .006". Many american makers though the shooter didn't truly know what they wanted & bored I/C barrels to 1/4 choke but marked them I/C anyway. In true terminology I/C is a lesser choke than is 1/4 & is just what its nae implies, a very slightly choked "Improved" cylinder. This was considered a big improvement over a true cylinder in soft shot & card/felt wad days.
