I was re-reading T. Carr's encyclopedic travel tips post, and came to the part about needing a SAPS Form 520 if traveling to or through Johannesburg. I'm flying Dulles to Jo'burg to Windhoek, with only a short layover in Jo. and assume I do not need a SAPS form. I am correct?

If you never leave the internatonal terminal and check your baggage straight thru to Windhoek you are correct.

Im also in that situation, thanks for the answer

Odin

Die Ou Jagter is correct. I did exactly what you are describing last month. I checked my guns through from the US to Harare, did not pick them up in RSA, and did not need a SAPS form. Similarly, I flew from Harare to Windhoek, through Jo'berg, checking the guns to Windhoek. Again, I did not need a SAPS form in Jo'berg, as I did not pick up the guns or leave the airport. And one last time, I flew from Windhoek, back to the states, with a short stop in Jo'berg. Again, the guns were checked all the way to the US from Windhoek, and I did not need any SAPS form in RSA.
Don't worry.
Bill

So that also means there is no point in hiring Optima or some other similar service?

As Die ou Jagter said if you do not leave the international airport terminal and made sure you checked luggage and rifles straight through to Windhoek you should be fine. No SAPS 520 or Optima if it is a straight forward connection flight. More important is to check you turn around times as these can sometimes be a pain.