But we have been stuck on standard 12ga, 16ga, 20ga, 28ga .410 for half a century
You are I think a century or so behind here. Most of these gauges were in existence prior to the introduction of breech loaders, with the possible exception of the .410. As I recall on the Lewis & Clarke expedition they carried one gun to be used with either ball or shot, which was essentially a 28ga, perhaps a little heavier in the bbl than a true shotgun.
In the late 1800's & early 1900's a myriad assortment of loads were available for shotgunners. Hull lengths were available for most of the gauges from 2½ up to 3¼" in 1/8" steps with a few having a 1/16" increment thrown in. For an order of as few as about 100 shells a buyer could specify virtualy whatever they desired in way of hull length, powder brand, type & amount, shot size & hardness type of wadding etc, etc. Far more shot sizes were then available. Around about the time of WWI, not sure of exact dates, the companies began streamlining & standardizing. It was all economics. Unless you are wuite willing to pay a considerably higher price than even the current prices, you should be thankful for the lack of variety. Mass loadings of a lesser variety gives us better prices "Comparitively speaking" than shooters of 100 yrs ago enjoyed.
