A unit of measure of the internal diameter of a shotgun barrel, determined by the number of spherical lead bullets of a diameter equal to that of the bore that are required to make one pound... According to Webster's New Universal Unabridged Dictionary

The exception to this rule of course is the .410 bore

The nominal bore of a .410 shotgun is.410 inch. Which is why it it ought to be called the .410 bore instead of the .410 gauge. If the .410 had been named in the traditional fashion, by the number of lead balls .41 inch in diameter needed to make one pound, it would be about a 67-68 gauge. Many years ago it was also called the 36 gauge, and I have seen a picture of an old box of Remington shells marked "(36 GA.) .410-2 1/2 IN. (12 MM)." However, the "36 Gauge" designation was very inaccurate, as a true 36 gauge gun would actually have a .506 inch bore diameter.

OK but I want to know who came up with such a goofball unit of measure....the BRITS?

You're right. It was the Brits.