One thing you will have to have is documentation on US Customs forms that you are the legal owner of the firearms. You will have to have this done at the local US Costoms office during regular buisness hours. If you don't you will have hassles getting into South Africa and back into the US.
Here is a link
http://www.cbp.gov/xp/cgov/travel/cl...gistration.xml
The only problems I had last September (in the middle of the transition) and just spending 1 night before heading to Zimbabwe was matching the numbers with the documentation. Not that big of a deal.