MoJo1;
That 3" length sounds like perhaps you may well be measuring to the beginning of the bore, IE, including the forcing cone. I have used the method Researcher described for many years & it is indeed quite accurate. Just remember when holding the bbls up to the light & looking through as he described the forcing cone is seen as a shadow. You want to then measure until the corner of the scale just touches the "Beginning" of the shadow, "Do Not" include the shadow portion & measure to the beginning of bore diameter.
Try it again & do several measurements to insure you are consistant. "IF" it indeed has 3" chambers I would for sure want a check on wall thickness over the chamber ends (never a bad idea anyway).
Many cut bbls have had the bead re-installed. Another clue, though not an absolute is whether the two bbls touch at the muzzle. They most often did from the factory but usually don't on cut bbls. A determination of both the constriction & length of the choke will be the more reliable method of determination.
I just pulled a dime from my pocket & measured its OD with a caliper & it read .705". Most full chokes of this era had at least .035" of constriction & usually around .040". To accept the dime & still be full choked would mean the bore would have an absolute minimum dia of .740". Not likely "Unless" it was originally bored purposely for all brass shells.
As mentioned by the previous poster this is the caveat of the "Dime". Not all bores are created equal & choke is not an absolute size but the amount of constriction. The dime will give a good indication in most bbls, but do be aware it is not 100% reliable.
