UPS ground, with out hazmat fee. This is how I received mine.
FedEx told me they wont do it.
And I think if you put live ammo in a flat rate box you will get into trouble.
Call UPS hazmat dept. for details, and about marking box ORM-D consumer commodity.
under 50cal. or 8gauge shotgun can be shipped for ground charges without hazmat charges.
and can only be shipped ground, no overnite or air.
Hope this helps