Handy - You are correct, an FFL can sell a rifle or shotgun to a resident of ANY state as long as the sale complies with the law of BOTH states. But the original question was a sale of a rifle between two non-FFLs across state lines. Any interstate sale of a firearm (either a handgun or rifle/shotgun) between two non-FFLs has to go through an FFL. Here's a summary of the federal law, though some states may be more restrictive. I put the state where the non-FFL individual may come from in parentheses.
INTERSTATE - longarms
FFL to FFL = OK
FFL to individual (any state) = OK
individual (any state) to FFL = OK
INTERSTATE - handguns
FFL to FFL = OK
FFL to individual (FFL's state) = OK
individual (any state) to FFL = OK
Here's the statute.
18 USC 922(a)
It shall be unlawful:
(3) for any person, other than a licensed importer, licensed manufacturer, licensed dealer, or licensed collector to transport into or receive in the State where he resides (or if the person is a corporation or other business entity, the State where it maintains a place of business) any firearm purchased or otherwise obtained by such person outside that State, except that this paragraph (A) shall not preclude any person who lawfully acquires a firearm by bequest or intestate succession in a State other than his State of residence from transporting the firearm into or receiving it in that State, if it is lawful for such person to purchase or possess such firearm in that State, (B) shall not apply to the transportation or receipt of a firearm obtained in conformity with subsection (b)(3) of this section, and (C) shall not apply to the transportation of any firearm acquired in any State prior to the effective date of this chapter;
The exception in (b)(3) is what protects an FFL selling rifles & shotguns interstate.
18 USC 922 (b)
It shall be unlawful for any licensed importer, licensed manufacturer, licensed dealer, or licensed collector to sell or deliver:
(3) any firearm to any person who the licensee knows or has reasonable cause to believe does not reside in (or if the person is a corporation or other business entity, does not maintain a place of business in) the State in which the licensee’s place of business is located, except that this paragraph (A) shall not apply to the sale or delivery of any rifle or shotgun to a resident of a State other than a State in which the licensee’s place of business is located if the transferee meets in person with the transferor to accomplish the transfer, and the sale, delivery, and receipt fully comply with the legal conditions of sale in both such States (and any licensed manufacturer, importer or dealer shall be presumed, for purposes of this subparagraph, in the absence of evidence to the contrary, to have had actual knowledge of the State laws and published ordinances of both States), and (B) shall not apply to the loan or rental of a firearm to any person for temporary use for lawful sporting purposes;
.