410 gauge is an exception to the 12-16-20 gauge scheme. The bore measures .410 inches in diameter, very roughly 28 gauge, I believe. Western Auto, like Sears, contracted for some of their guns from manufacturers, such as Savage or Marlin. Check into some of the gun forums - I am sure someone there will know for sure
How about 67.5 gaauge?
http://en.wikipedia.org/wiki/Gauge_(bore_diameter)