in fact, the .410 bore is equivalent to a 67 gauge


https://www.americanrifleman.org/articles/2017/12/6/rifleman-q-a-is-410-a-gauge/



In 1961, the Permanent International Commission for the Proof of Small Arms (generally known as C.I.P.) finally settled on a designation and standardized measurements for the .410, reversing its earlier decision to use the inaccurate description of 36 gauge (it would actually be a 67 gauge).

It was not to last. The dithering by the Commission, for over 70 years, as to what the official name for the gauge should be had lead to the long-term adoption of the terms 36 gauge for the 2" & 2 shells and 36 gauge magnum for 3" shells in mainland Europe and these were too well established in the minds of users to be changed arbitrarily. In 1969, the name 36 gauge was appended to the official name in parentheses and thus it has remained.

http://www.smallboreshotguns.com/410-bore/


Last edited by skeettx; 01/13/19 07:54 PM.
USAF RET 1971-95 [Linked Image from jpgbox.com]