Argo;
Are these guns True 36 gauge?
I know that .410's were sometimes labeled both as 12mm as well as 36 gauge, both of which are incorrect. A .410 bore would be 10.41 mm, the 12 mm is approximately chamber diameter on the .410, not bore size. A 36 gauge is a nominal .506" (12.85 mm) diameter. I think a few were made, though not common. The 32 gauge is found more often with a nominal bore of .526" (13.36 mm)

Miller/TN
I Didn't Say Everything I Said, Yogi Berra