I copy/pasted this directly from Midwest Express's website:
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Passengers may not carry firearms and ammunition aboard as carry-on luggage. However, unloaded firearms may be transported in checked luggage if declared to the agent at check-in and packed in a suitable container. Handguns must be in a locked container. If you are bringing small arms ammunition for personal use, the ammunition must be in the original box and may be transported in checked luggage. Each passenger is limited to a maximum of 11 pounds of ammunition in checked baggage.
The following is a summary of key regulatory requirements to transport firearms, firearm parts or ammunition in checked baggage according to the Transportation Security Administration’s policies:
• All firearms must be declared to the air carrier during the ticket counter check-in process.
• The firearm must be unloaded.
• The firearm must be carried in a hard-sided container.
• The container must be locked.
• It is preferred that the passenger provide the key or combination to the screener if it is necessary to open the container, and then remain present during screening to take back possession of the key after the container is cleared. If you are not present and the screener must open the container, the TSA and/or airline will make a reasonable attempt to contact the passenger. If this is unsuccessful, the container will not be placed on the plane since unlocked gun cases (or cases with broken locks) are not permitted on aircraft due to Federal regulations.
• Any ammunition transported must be securely packed in fiber (such as cardboard), wood or metal boxes or other packaging specifically designed to carry small amounts of ammunition.
• Firearm magazines/clips do not satisfy the packaging requirement unless they provide a complete and secure enclosure of the ammunition (e.g., by securely covering the exposed portions of the magazine or by securely placing the magazine in a pouch, holder, holster or lanyard).
• The ammunition may also be located in the same hard-sided case as the firearm, as long as it is properly packed as described above.
• Black powder and percussion caps used with black-powder type firearms are not permitted in carry-on or checked baggage.
The regulations are strictly enforced. Violations can result in criminal prosecution and the imposition of civil penalties of up to $10,000 per violation.
Please visit TSA.gov for further information regarding the transportation of special items.