There should be a marking on each barrel showing bore size in mm's at time of proof. "Nominal" 16 ga size (.662") would be 16.8 & may use a comma rather than decimal point. 16.8/25.4 gives .661" while 16.9/25.4 gives .665". You can compare the marking with current bore size to determine the extent, if any, of which it may have been honed.
As Drew mentioned the C has no reference at all to "Cordite", in fact it is very doubtful if cordite was ever used in a shotgun, it being primarily a Rifle propellant. Barrels of this era were marked for both bore size & Chamber "C" size. You may find guns with a 16 chamber having bore sizes from 15 to 17 or more.
