There is no difference with a locked case outside a suitcase.
Nice to know the posted airlines' websites are wrong as well.
1.Firearms must be unloaded and packed in (1) a manufacturer's crush-proof type container, manufactured specifically for the firearm or (2) a hard case. Handguns checked as baggage must be contained in a hard-sided, lockable case, or in a hard-sided container inside a soft-sided case. Firearm containers must lock with a key or lock combination that remains in possession of the customer and must be locked by the passenger in the presence of AS. No exceptions. Baggage containing firearms will be transported in an area, other than the cockpit, that is inaccessible to the passenger.
2.Baggage containing firearms will not knowingly be accepted for transportation at point of origin or at an online or interline connecting point unless a declaration, signed and dated on the day the baggage is accepted for transportation, is placed inside the case containing the firearms, declaring that the firearms are not loaded.
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Baggage containing firearms must be locked at all times and the key or lock combination retained by the passenger.
A Firearm Unloaded Declaration form (available only at the airport) must be signed and placed inside the bag or gun case.
as stated in an earlier post, I fly Southwest only... and made an assumption that most would be simliar... obviously incorrectly... so definitely check your airline because it appears that what one airline requires another adamantly rejects....