Hi, Tony,
I hope you don't mind a minor nitpick. You say, "The bullet diameter therefore needs to be in between the diameter of the bore and that of the rifling, so that it is gripped firmly."
In a conventional breech loading rifle, the bullet diameter is the SAME as the groove diameter; the bullet is forced, by the pressure, through the barrel and the rifling lands (bore diameter) are engraved into it.
So using the .30 as an example, and ignoring manufacturing tolerances, the bore diameter is .300, the groove diameter is .308, and the bullet diameter (before firing) is also .308. Hope this helps.
(I used the term "conventional" because there have been rifles and handguns made with very deep rifling to allow some of the powder gas to blow out around the bullet to relieve pressure and allow powerful ammunition to be used in a firearm that otherwise could not handle the pressure, such as a .45 ACP in a blowback pistol. These barrels don't last long due to erosion, but achieve the purpose.)
Jim