Tony, the back-boring or enlarging of the normally .729" 12 bore, up to .740" appears to be common today.
The rifled barrel of my Mossberg M835 had a .724" groove diameter, however the shotgun barrel's bore is .735".
This gun, "came" with 2 barrels, one 24" and rifled, while the other was 28" with rib, screw-in chokes, both for steel and lead and was smooth.
The rifled barrel shoots very well with 1 ounce 3" Winchester Fosters @ 1,750fps & 2 3/4" Cdn. made Challenger Ammo with 496gr. Gualandi DGS slugs at 1,610fps.

About the video on .690" ball casting and shooting. I have single cavity Tanner moulds, now made by Jeff's son since he died, also have a large casting sprue. Instead of side cutters, which the fellow called flush cutters, I use the pliers type of wire strippers. Using one of the small cut-outs, I cut in about 1/2 way through the sprue then twist or rotate the ball. This ends up cutting completely through the sprue's stem and leaves the tiniest little tit sticking up. One tap with the jaws of the cutter and it is now flush with the top of the ball- which then appears perfectly round, without the flat spot left by side (flush) cutters.
33gr. Long Shot is not maximum. I see data for up to 36gr. with a 525gr. sabot slug in a field wad.
As the big guy said, these work best.
I haven't watched the video all the way through yet, but expect the speed to be in the 1,400fps range.