Plugs are fine but can be misleading , some older chambers may be a little tighter than modern ones so a modern plug will no go in fully . I have had this on numerous 12 bores when the plug would not go all the way to the bottom of the chamber , but a couple of turns of a new chamber reamer cleaned them out with out deepening . Eye and a steel rule should give you a fairly close reading depending on the cones. Guns of this period made for roll turnover cases often had much shorter cones .