Geno is correct about the measurements for a true 36ga. The problem is that a .410 as Miller points out has a head diameter that is close to 12mm. So there has been much confusion.

You can see that Magtech continues with the confusion by designating the .410 as 36ga. The 410 is in fact a 68ga.
http://www.cabelas.com/p-0031462215796a.shtml

At one point CIP designated the .410 as 36ga.
http://cartridgecollectors.org/410/collecting410.htm

For the most part, the American path to the .410 was the 44XL. I may have seen some early American, 1900-1910, single shots stamped 12mm, but never 36ga. The early pin fire Lefaucheux and Italian Marietta pistols were designated as 12mm.

Pete

http://www.damascus-barrels.com/Movie.html