12 mm = .47244" . This, of course, is not the bore size of a .410 shotgun but would fall within the chamber dimensions.
As stated 36 gauge = .506", obviously not a .410.
a .410" bore size per both British & German proof charts = 67.49 gauge.
Some gauge charts only go down to 50 (.453") but there are others, more complete, which continue @ .45" & go down to .30" in .010" increments. these are;
.45" = 51.04 Ga
.44" = 54.6 Ga
.43" = 58.5 Ga
.42" = 62.78 Ga
.41" = 67.49 Ga
.40" = 72.68 Ga
.39" = 78.41 Ga
.38" = 84.77 Ga
.37" = 91.83 Ga
.36" = 99.7 Gauge
.35" = 108.49 Ga
.34" = 118.35 Ga
.33" = 129.47 Ga
.32" = 141.95 Ga
.31" = 156.14 Ga
.30" = 172.28 Ga
This could be extended further as;
7 mm = .276" = 221.24 Ga
6.5mm = .256" = 277.25 Ga
6mm = .236" = 353.88 Ga
5.6mm - .220" = 436.84 Ga.
You can take 16 & divide by any gauge no & it will give the weight of the round ball in ounces or divide by 7000 & it will give the weight in grains.
Probably more than anyone wanted to know, but that's the scoop.
Regardless of how widely used or how much accepted either 12mm or 36 gauge are total MISNOMERS, for a gun with a .410" bore. either smooth or rifled. In metric its a 10.41. Remember the old 10.4mm Swiss Vetterli rimfire (.41 cal) while in gauge it is as stated a 67.49.
I could be wrong but my personal belief is the original promoters of it just didn't want people to truly realize just how LITTLE the thing actually was. 36 gauge sounded bigger than a 67.49 & 12mm sounded bigger than a 10.4mm.
