Hello Mark;
To recap what you probably already know, my understanding is that for South Africa, at the current time (circa August 2018):
1) South Africa allows one person to import up to 200 rounds of ammo per firearm (maximum of 4 firearms, but not more than 1 firearm per caliber);
2) All airlines limit the ammo to 11 lbs (in practical terms this includes box & TSA lock, although it is not stated per se);
3) South Africa allows leaving the country with unfired ammo;
4) All South Africa airlines requires ammo to be checked separately from checked luggage on internal flights;
5) South African Airlines and US airlines allow ammo to be included inside checked luggage (not in the rifle case obviously) for international flights.
One is likely to reach 11 lbs, before the 200 rounds per gun count - unless bringing 22 lr ;-) I use a MTM Travel-Survivor dry box (size large) for the ammo, which I put in my duffel bag for international flights, and check separately in South Africa. Note the two empty slots. This resulted from weighing the box (
including TSA lock) to come in at just below 11 lbs.
Following these generalities, and to answer specifically your question:
1)
A person is legally prohibited from importing ammo for a rifle that this person is not importing. The SAPS 520 in this person's name will be considered proof of import. The firearms listed on the SAPS 520 much match those listed on the US Form 4457 in this person's name. Therefore, if my reading of the regulations is correct (?), your plan is TECHNICALLY ILLEGAL.
2) I have personally never seen an airline/border protection/immigration/airport security/police agent actually weighing or opening an ammo box, much less counting rounds or verifying cartridge headstamps. Therefore your plan COULD work.
3) There have been folks who reported being prevented from importing reloaded ammo with headtsamp not matching the caliber stamped on the barrel of their rifle. I am therefore assuming that a verification CAN happen.
4) I would personally not take the risk you are contemplating, as I see so many more potential downsides than upsides.
To avoid the painless arguing at customs, police, security, etc. I actually use Afton House to secure a pre-approved SAPS 520 which includes the number of rounds imported for each rifle, and to walk me at the international airport through the various customs, police, security, etc. upon arrival in SA.
I believe that it is $100 well spent (even if only in time saved) because I really have little patience for the haggling with misinformed or tip-chasing various "officials" and I HAVE been asked very directly for a tip by EVERY person who had to fill up paperwork for my rifles and ammo when I processed them alone without Afton in SA in August 2018 (arrival to and departure from East London, and departure from 'Joburg'). Everything went well but,
bottom line, I would not even dream of putting myself willingly outside of the regulations/laws with firearms & ammo anywhere in Africa...
Just my $0.02
Have a great trip.
PS: take a stack of $5 bills to tip officials asking for it (even though it is technically illegal for them to do so), as $1 is not enough (they told me contemptuously), and I would certainly not tip $10 or $20 for the privilege of watching someone actually do nothing but their paid paperwork job!)...