Backboring is a term that is mostly misunderstood as being the forcing cone due to the fact that when working on the forcing cone area you are working on the back end of the barrel then afterwards the barrel is supposedly ''backbored'' but in reality it is the actual bore of the gun from just ahead of the forcing cone all the way through the barrel to the choke.
When you backbore a barrel you are enlarging the bore size of the barrel. When this is done it does several things: It usually is done to increase the amount of choke with a given choke diameter.
A back-bored barrel is a shotgun barrel that has a bore diameter that is increased beyond the minimum specifications. Increasing the bore diameter of a shotgun barrel greatly enhances its performance. As a larger bore diameter reduces friction of the shot charge against the barrel wall. Instead of trying to overcome friction, powder gases expend more energy on the wad base, resulting in an increase in shot velocity. Because there is less constriction or pressure from the barrel walls on a shot charge, there are fewer deformed shot pellets. A greater number of ballistically superior round pellets in a shot charge gives you exceptionally uniform patterns -- delivering more shot pellets in the effective part of your pattern.