I would go to the post office a couple of days in advance of the shipment and speaking to the personnel at that office regarding how they want to handle it. Bring a printout of the regulation as you cited it -- do not expect them to know that you can legally mail a rifle.
Once you get it established that what you want to do IS legal, then discuss this part:
The Postal Service may require the mailer to open the parcel or give written certification that the weapon is unloaded and not concealable.
Get a clear understanding up front as to whether or not they will require you to open the package so they can verify that it's a rifle and that it's not loaded, or if they'll accept your certification. If they are going to require that they verify the contents, then there's no point in doing a super packaging job at home. Put the rifle in a sturdy box, and bring your address labels and lots of heavy packing tape with you to the post office.