Here is a link to Spirit Airlines Firearms requirments
Conditions for Acceptance of Special Items
The following are special items that will be accepted as checked or carry-on baggage, subject to specified conditions and payment of charges when applicable.
In accordance with Federal law, a passenger who presents baggage containing a firearm must sign a declaration that the firearm is unloaded and packed in a suitable container before such baggage will be checked. Ammunition must be securely packed in durable fiber, wood, or metal boxes or in the manufacturer's original container. The quantity of ammunition is subject to regulatory limitations. Baggage containing style="COLOR: black; BACKGROUND-COLOR: #ff66ff"handguns must be locked with a key or combination lock in possession of the passenger only, and the bag must be of the hard-side type.
Carriage of firearms and/or ammunition on international flights is strictly prohibited unless all domestic and foreign regulations are adhered to. Specific requirements are determined by the foreign authorities and differ based on the destination.