Neck turning
Using a neck turning tool to reduce the neck wall thickness to a specified amount. This is not usually done for a factory barrel and chamber.With a factory chamber the tool can be used to clean up 80 or 90 percent of the neck wall so the wall thickness is even so that the bullet will be somewhat centered. If you measure neck wall thickness at different places around the neck and get different thicknesses then you can remove just enough material to even the thickness out.
Some target shooters turn their necks to a certain thickness so that each cartridge requires the same amount of pressure to release the bullet when the gun is fired. Some have their rifles set up for a no turn neck. This is why I marked neck turning as optional, take your pick.