The allowable min. / max. tolerances for the bore of a barrel to be designated as a "12 gauge" (or any gauge for that matter) have always been horribly loose. Take a look:

Yes, your eyes are working correctly, the 1954 British "standard" allows anything from a .710 in. inside diameter to .750 to be called a "12 gauge." No modern shotguns I'm aware of vary from gun to gun nearly that much; however it sure pays to measure your bore as many guns do stray from the touted .725 inch "European standard" and the .729 inch "American standard" for 12 gauge.

Backboring (overbore and underbore barrels) have been experimented with for a long time. It was around 75 years ago that the 12 gauge "3 inch magnum" shell was developed. Shotgun barrel bores were intentionally opened up to the British maximum of .750 in. at that time, with the press back in the day bragging of the lower recoil and better patterning that was observed.

They had it right, but only half right. Anytime a shotgun barrel is increased in inside diameter with the exact same shotshell, both pressure and velocity are reduced due to the increased barrel volume. There is less recoil, but only because muzzle velocity drops. Pattern densities may often improve, but again due primarily to lower velocities that deform less lead shot on initial setback. You can accomplish the same in any shotgun by lowering the muzzle velocity, all other things remaining constant.

That why touting "backboring" remains one of the pet rocks of shotgunning today. The real reason to backbore is to pull weight out of a heavy set of barrels. Any other significant "benefits" were disproved well over half a century ago.