Would be best if you were going to lap the barel, to just do the whole thing. I dont really quite understand why only the last portion would be done. Also, depending on the qaulity of the barell, it might not even need to be lapped at all. An old barell with many rounds shot through it might require a lapping. A brand new schneider or shilen more then likely wont. Purpose of lappings as John has mentioned is to smooth out the barell so that it is easier to clean and clears out whatever tool marks are left in the barell so that when the round is fired it wont nick and burr the bullet. It will have a smooth travel. Tubbs Final Finish I hear is a good product to do the lapping with.
Respectfully,
Tyler Consugar