bersaguy, your question is fascinating.
I'm not sure you're on the right track, and I'm not sure you're on the wrong one, either.
Recall all the Kentucky or Pennsylvania muzzle loading rifles of the late 18th and 19th Centuries were "36" caliber. "40", "45" and "50" calibers were popular as well. From where was the criteria for any caliber derived?
During the age of black powder, larger bore diameters were found to be useful as muzzle velocity was limited. With the upper limit of velocity (due to the burning characteristics of BP) the only way to get more power from a "36 caliber ball" was to fire a "37 caliber ball" at the same maximum velocity. Or a 40 caliber or bigger, like 69 caliber.
With the technology of measurement and machining at the time, the 69 caliber (used in many military muskets) was pretty much a suggestion. Since the projectiles were typically pure lead (or close enough) anything near enough to stuff down the bore was 'good enough'. Many early handguns (single shot and repeater) were much the same; even now a soft .358" bullet will be sized in the process of shooting through a .356" barrel. (Such a bullet turned from tool steel might be uncomfortable, but it doesn't occur often.)
I have not researched the date of discovery, but I'm reasonable sure 'conical bullets' (not round balls, but longer, pointy on the front, flatish on the back) became available prior to the U. S. Civil War. This added length, adding weight (more lead) to the projectile, so one could have bullets of more weight than that of round balls. Resulting in more kinetic and momentum energy. In the same era (generally) measurement was developed. Then with the change to cartridge ammo and the alteration from 'outside lubrication' to 'inside lubrication' the gun builders started understanding that many commonly named cartridges were misnamed on a technicality.
So the .36 caliber cap and ball revolvers evolved in the .38 caliber handguns. Or in Europe to 9mm. 44 Special (and Magnum) are .429" but the .44-40 or .44 Winchester Central Fire (.44 WCF) are officially sized .427". .45 Colt revolver was set at .454" initially, and the later .45 ACP at .452" (.451" in some minds); the .45-70 Springfield rifle and the later .458 Winchester Magnum are .458".
The 'guage' method you mention was generally phased out. There are still shotguns identified and called "[something] guage" but all the plans and blue prints have specific English or Metric measurements. And the loads no longer have much semblance to 1/12th, or 1/20th of a pound of lead.
Seems to me all relation to the older gauge system is gone.
Revolvers and handguns in general have larger bore diameters as a heavier bullet can be used. And a .32 bore handgun (.32 ACP for example) is roughly the same diameter as .30 caliber rifles. Even a .30 caliber Speer 100 grain half jacket bullet is just barely - if at all - capable to be loaded into a .32 ACP case with a powder charge and then be loaded in a pistol magazine. Frankly, it would not be possessed of any useful velocity or power. Something in the .357 Magnum category is probably minimal for self defense in all cases and many think the blast and recoil are severe.
Now, back on track; a .36 caliber black powder revolver is not by any stretch a raging power house. It did have more reach than a Bowie knife but was less effective than the knife in application.
Always fascinating to consider "What were they thinking?"
