Were going to Vic Falls through JHB RSA on the 17th. We have locking ammo boxes. Can we put the boxes in our checked bags or do we have to use them as additional checked baggage?

I flew out of Joburgh via Virgin last week with ammo in my suitacase in a locked steel box. All declared at check in with no problems.
There is a firearms check in after passport control and, once you've checked in with your airline, you have to take your firearms and import licenses there to clear them out of the country.