You don't need the tap, although it would be good, you use the thread restorer file to carefully restore the threads on the action(inside threads) and the old barrel (outside threads) so they are both good again.

If you don't want to spend the time with the file, buy a tap, but for the best repair job, make sure it is a thread restoring tap, which is designed to reshape the metal back into the correct form, as opposed to a regular or cutting tap, which will cut off anything except the thread, thus leaving the thread not fully formed (usually not as deep) in the areas where it was scarred up.