Even though we are talking primarily about shipping firearms, here is a link on an article about shipping ammo. http://www.adazonusa.com/howtoshipammunitionorammo-a-45.html
The article states thus(and I checked the DOT as well);
"According to UPS, shipping ammunition is allowed, but there are some stipulations. The stipulations are not put in place by UPS, but instead, by the Department of Transportation. The D.O.T. has a classification called Title 49 CFR that spells out the details. We have summarized those details for you specific to shipping ammunition. FedEx has the same requirements, but also wants you to include a “Shippers Declaration for Dangerous Goods” form which is on their site if you do a search for “Dangerous Goods Forms”. The USPS follows the D.O.T. requirements but in talking to some of the USPS locations, the managers stated they would not accept the packages anyway in this day and age, so we recommend only using UPS or FedEx to ship ammo."
Technically it is not illegal to ship ammo USPS, they just won't do it. Also, as I stated before, to ship ammo Fedex ground you do not need to fill out the mentioned form. It is for air shipments only. That was told to me directly from Fedex Hazmat help desk.