Been playing this through in my mind...
Say legislation is introduced, mandating that full faith and credit be applied to CCW permits. Trump signs it into law. I'm betting that the AGs in California and/pr New York and/or Massachusetts would file a suit in federal court seeking a declaratory judgment that such legislation is unconstitutional because (1) no authority under the Constitution, (2) state's rights, and (3) any other thing that they can get some legal intern to dredge up. Odds are good in one of those states, they'll get an anti-gun judge who will find such a law unconstitutional, especially in New York or California. This gets appealed, and chased up to SCOTUS. Depending on how fast it happens, it may still be a 4-4 vote in SCOTUS, which would mean that the lower court findings stand, and there would then be legal precedent that full faith and credit does not apply to CCW permits. Not a good outcome.
Other option. Law passes, and Johnny Gunner goes to NYC packing his Glock 19. NYPD arrests him for CCW, illegal possession of a firearm, and probably violation of the New York SAFE act which prohibits mags with more than 10 rounds. NYC judge (probably anti-gun) finds the federal law unconstitutional and Johnny gets convicted and sentenced to prison. Johnny then chases the case through the appellate process, which would likely take a year or two in New York. Meanwhile, maybe Trump gets an appointment to SCOTUS, and maybe Johnny gets his conviction reversed and remanded, which means it goes all the way back to the trial court. Local judge dismisses the CCW and possession charges, but maintains the SAFE act conviction and keeps Johnny in prison. Better outcome for all of us, but sucks to be Johnny.
As I've been thinking this through, especially with different states having mag capacity limits, ammunition restrictions (aren't hollowpoints still illegal in New Jersey?), and such, should full faith and credit eventually get passed, I think I'll go find a 3" Smith Model 13/65 and make that my travel CCW gun, loaded with LSWC ammunition. No worries about magazine limits, or silly ammunition restrictions, or some AG like in Massachusetts who interprets local assault weapons bans to essentially include any semi-auto firearms capable of using a detachable magazine which might possibly hold more than 10 rounds.