Freebore in the Weatherby Factory rifle gives the bullet a longer distance before it contacts the rifling. The bullet acually jumps to the rifling. Benchrest shooters long ago found that a rifle is much more accurate when the bullet starts out just touching the lands of the rifling or not more then .010" to .015" inch from the lands.
Most Weatherby rifles you cant touch the rifling even if you just barely seated the bullet in the neck 1/8".
It was devised to give a little bit more velocity but, at the expense of accuracy.
Most Weatherby factory rifles do not hold very good accuracy as compared to standard "Normal" chamberings.
You could have a good gunsmith chamber a 340 Weatherby without freebore on a custom barrel and it would be a much more accurate rifle in most every case.