Realunlucky. TSA does not have any rules that specify only one box of ammo. If so it's brand new. I've always gone by the more common 11-12 pounds of ammo. The TSA agents like to see it packed in factory ammo cardboard box. But plastic ammo boxes should be legal as well as long as it was designed to hold ammo (ie the container keeps each round from contacting each of the other rounds).
This is from Delta Airlines regarding checked baggage on "shooting equipment"
Shooting Equipment
Shooting equipment is allowed as checked baggage only. It must fit within the very specific criteria that we outline below.
Declare to the Delta representative that you are checking a firearm.
Declare the existence of a firearm to security personnel if there's a security checkpoint before the Delta counter.
All firearms must be declared by the passenger to a Delta representative at the main ticket counter.
Present firearm(s) unloaded and sign a "Firearms Unloaded" declaration.
Firearms must be packed in a locked manufacturer's hard-sided container specifically designed for the firearm, a locked hard–sided gun case or a locked hard-sided piece of luggage. Handguns may be packed in a locked hard-sided gun case, and then packed inside an unlocked soft-sided piece of luggage. However, a Conditional Acceptance Tag must be used in this case.
Maintain entry permits in your possession for the country or countries of destination or transit.
Ensure small arms ammunition is packed in the manufacturer's original package or securely packed in fiber, wood, plastic or metal boxes and provide separation for cartridges.
You are responsible for knowledge of and compliance with all Federal, State or local laws regarding the possession and transportation of firearms. For more information about this regulation you can visit the TSA website.
If you are transporting a firearm to the United Kingdom, a permit from the United Kingdom is specifically required. You must contact the United Kingdom for more information about securing this permit.
The following types of ammunition are not accepted:
Gun powder; such as Pyrodex or Black Powder
Ammunition with explosive or incendiary projectiles
Ammunition exceeding 11 pounds (5 kg) gross weight per passenger
Pistols and accessories must be included in one case and contain:
Pistol telescopes
Noise suppressors
A small pistol tool case
No more than 11 lbs. (5 kg) of ammunition
Rifles and shotguns must be packaged as follows:
One hard sided case containing up to four rifles, shotguns, shooting materials, tools
One hard sided case containing up to five handguns, one scope, tools
One bow and quiver of arrows and maintenance kit enclosed in a case or container strong enough to protect the bow and quiver from accidental damage
No more than 11 lbs. (5 kg) of ammunition
An excess baggage fee will apply if checking more than one gun case.