Check your airline's website for written instruction on traveling with firearms. Each may all different amounts (weight) of ammo.
Also, check with the state that you're traveling to and abide by their rules on how to legally travel/transport a firearm in their state.
I found this on the Spirit Airlines website:
"6.4.1.16. Firearms
Spirit Airlines will allow customers to transport firearms in accordance with applicable regulations. Except for personnel authorized by the federal government, the following conditions must be met:
a) Firearms are accepted as checked baggage only and must be declared to an agent at check-in.
b) No one under 18 years of age may check a firearm. A customer who presents baggage containing a firearm must declare that the weapon is unloaded and will be required to sign a Firearm Declaration tag.
c) Firearms must be packed in a hard-sided container specifically designed for the firearm; a locked hard-sided gun case; a locked hard-sided piece of luggage.
d) A handgun packed in a hard-sided gun case may be packed inside a soft-sided piece of luggage.
e) Firearms may not be checked curbside.
f) Baggage containing firearms must be locked at all times and the key or lock combination retained by the customer.
g) If the firearm in the hard-sided, locked container is inside a piece of checked baggage, the Firearm Declaration Tag will be placed inside the checked baggage and proximate to, but not inside of, the hard-sided, locked container.
h) If the firearm is not inside another piece of checked baggage (for example: a rifle case), the Firearm Declaration Tag will be placed inside the hard-sided, locked container.
i) Ammunition must be packed in the original manufacturing package or metal/wood box, with a maximum of 11 pounds per customer.
j) Loose ammunition or clips will not be accepted.
NOTE: Firearms are NOT permitted to/from international destinations."
Also, you can read the TSA guidelines on traveling with Firearms and Ammunition on the TSA website:
Firearms and Ammunition | Transportation Security Administration
Be safe and good luck!