Thanx for vaidating that, but I think you really mean 'ballseat' (the reduced dia. ahead of the chamber proper) where the bullet is actually supported before hitting the rifling. The freebore, when done properly, is in the length of the ballseat, and, just like it sounds, is essentially the bore free of rifling. Weatherby has been using freebore effectively for years.
Technical semantics aside, these concepts are what we use as tools as reloaders because we can fine tune a cartridge to best match our chambers, and thats what can make the magic happen.