David;
I do not know if it is true or not, but the story I have heard on the 4 gauge guns was that when breech loaders first began to be made the bore was kept true to size but the chamber was cut for a thin brass shell. As Paper hulls began to replace the brass ones the chamber remained un changed with the bore size being dropped to accomadate the smaller wads. If made in a country with proof laws the barrel should be marked with he actual gauge of the bore. No inbetween sizes were set up for these larger gauges. Therefore a bore which would accept a .919" gage but refused a .976" one would be stamped as a 6. I have not had opportunity to examine many 4 gauges, in fact have only even seen a very few, but have been told that most of the British built ones indeed carry the 6 mark in their proofs. "True" gauge is of course determined by the number of ball which weigh one pound. By the formula used for this calculation a 4 gauge ball would measure 1.052" in diameter.
It is also noted that in the past many makers of .410's as either 36 gauge or 12mm. Neither is correct & they are not the same. 36 gauge has a diameter of .506" & 12mm converts to .472". The "True" size of a .410 is of course just that .410". These guns though were not made those sizes, just marked that way.
