Nominal bore sizes for 12 bore is .727" to .730". Today, some are over-bored to about .735" or so to reduce recoil but mainly in the trap guns. .760" is just over 11 bore as 10 gauge is .77 calibre.
: I think the fellow from NY was talking about rifled shotguns, not a smoothbored gun. In any event, 1.5" at 100yards is usually a good group for an off-the-shelf .30/06 with a 4X scope with factory ammo, let alone a sabot'd shotgun slug. I am not saying some rifled barrels aren't capable, just that I've just never observed it done. I have seen 2-1/2" and 3" groups at 100yards from sabot'd slug laods in rifled 12 bore barrels mounted with scopes.
; The sabots generally don't do well in a smoothbore in my experience. The old=style BRI's with the 440gr. hour-glass shaped slug generally show tipping at 25yds, let alone 50yards, but they do shoot wel in some guns. Their only down-side (except for innacuracy in mnay smoothbores, is the reduction to .50 calibre.
: Be aware that the double shotgun adds regulation to the equation and many of today's shotguns may have too much convergence of the barrels to shoot a variety of loads. I was lucky with mine, perhaps as the breeches weren't overly wide. As well, the double's tapered barrels give quite a bit of elevation to the breech and has to be taken into consideration with iron sights.