Gauge is simply the measure of "Thickness of an Object". This can be shotgun shells or a wire or tube. The lower the "whole" number the thicker/bigger the diameter. Example: 8 Gauge is larger/thicker than a 12 Gauge etc. ***Example as applied to "Shotguns" here*** : google.com/search?biw=1280&bih=618&tbm=isch&sa=1&ei=-5HcWuD0F8HuzgLwkpvoCA&q=gauge+size+difference&oq=gauge+size+difference&gs_l=psy-ab.3...19500.24900.0.25288.15.13.0.2.2.0.148.968.11j2.13.0....0...1c.1.64.psy-ab..0.7.478...0j0i8i30k1j0i24k1.0.mqDLwCDFK_Y#imgrc=MNJYgC31FSlRjM:

Gauge is simply the measure of "Thickness of an Object". This can be shotgun shells or a wire or tube. The lower the "whole" number the thicker/bigger the diameter. Example: 8 Gauge is larger/thicker than a 12 Gauge etc. ***Example as applied to "Shotguns" here*** : google.com/search?biw=1280&bih=618&tbm=isch&sa=1&ei=-5HcWuD0F8HuzgLwkpvoCA&q=gauge+size+difference&oq=gauge+size+difference&gs_l=psy-ab.3...19500.24900.0.25288.15.13.0.2.2.0.148.968.11j2.13.0....0...1c.1.64.psy-ab..0.7.478...0j0i8i30k1j0i24k1.0.mqDLwCDFK_Y#imgrc=MNJYgC31FSlRjM:

12 ga = 0.73 or 12 lead balls
16 ga = 0.66 or 16 lead balls
20 ga = 0.615 or 20 lead balls
28 Ga = 0.55 or 28 lead balls
68 Ga = 0.410 or 68 lead balls this is the .410 shotgun

The .410 is not a Gauge but a bore size it would be a 68 gauge shotgun

Gauge is a term to describe the Diameter of a shotgun barrel it is the number of round led balls of the bore size that equal 1 lb of lead. so 12 round balls of the diameter
of a 12 gauge shotgun barrel equal 1 lb of lead