It's all a mixed up deal. British proof houses accept .405" to .415 as the range of bore sizes for a .410. It takes 67 balls of .410 diameter to be a pound. Thus, it is truly a 67 gauge.
There actually is a 36 gauge, at .506" bore diameter, and Eley loaded shells for it. There's a picture of a box of them in Gabriel's .410 book. There was also an even smaller than .410" chambering ......... a .360" cartridge of 1 3/4" length and carrying 3/16 oz. of shot.
So, how the .410 ever got the mistaken identity of 36 gauge in the first place is a mystery to me. Same reason a largemouth bass is still called a "trout" in parts of the Deep South, I guess.
SRH
