Divide those numbers by 252 and you get the units I used in my posts above. His units are gn water per sq in of bore. Converting the former to cu in leaves you "inches of bore," a concept easier to visualize. He really should, though, use net case capacity, not empty case capacity.
This ratio is indeed important in internal ballistics and is one of two ratios (the other is SD) which largely determine the needed powder speed.
This version of the Powley Computer calculates that ratio.
His criteria for overbore is whether the cartridge is a "barrel burner." His number, though, fails to account for operating pressure which in turn determines the temperature of the gases. Lumping the medium pressure .257 Roberts in with the high pressure .22-250 seems a bit of a stretch.
If you assume for all cartridges the modern peak pressure limit of about 64,000 psi, you have a reasonable criteria for "overbore."
However, instead of using the vague term "overbore," why not simply call it a "barrel burner" directly? I'll note this fellow admits all "overbore" cartridges aren't barrel burners. Again, pressure and powder characteristics count.
While that definition of "overbore" is certainly workable, I still prefer the definition I listed earlier, one I recall I saw first from Ken Howell: it's overbore if the case is so large you can't find a powder to safely fill it.
Under this definition, "overbore" doesn't mean "barrel burner," it means "uselessly large." The .22-06 is large and likely a barrel burner, but it will indeed give you higher velocities than smaller .22's.
