I wouldn't risk it unless it was as simple as flying Delta directly to to JNB and nothing else.
On my last trip to Mozambique, in JNB I had to take my ammo out of checked baggage and take it as a separate item.
They don't care about the flimliness of "lockable box."
I just use the cheapest, thin metal, lockable, cash box that I can find, and it's never been a problem.
The lighter the box, the more ammo you can put in it without worrying about going over the limit.
It's as easy to steal a $100 Pelican as it is a $10 Walmart special.