Johannesburg — South Africa, put on a forced armaments diet by an international weapons embargo five years ago, has seen its own arms industry grow big and strong - and perhaps a bit overweight.
To get rid of some of the ''excess'' profitably, Pretoria has embarked on a major arms export program.
The reason for South Africa's push to sell more arms is mainly financial. A serious economic downturn here is forcing the government to either curb its large and costly munitions production, or sell more of it to other nations. Pretoria - which feels increasingly threatened by black Marxist neighbors - finds the sales option preferable.
Exporting arms is not a new enterprise for South Africa. But suddenly the government's munitions manufacturer, the Armaments Corporation of South Africa (Armscor), is trumpeting its export ambitions with an uncharacteristic openness that is probably indicative of the new seriousness with which Pretoria is seeking export sales.
''They are printing slick color brochures of things they wouldn't even used to talk about,'' says one knowledgeable source.
Armscor recently unveiled at a ''champagne breakfast'' for the foreign press what may be its star attraction on the foreign market - the G-6 mobile artillery unit. It puts one of the world's most accurate and longest-range guns on top of an exceptionally mobile vehicle, ushering in what Armscor unabashedly calls a ''new era in artillery warfare.''
Speeding along at 60 m.p.h. on a paved track at the Armscor testing grounds outside Pretoria, the G-6 displayed the quickness and mobility that its designers claim make it unique. Its six-wheel chassis can move it speedily through rough, bush terrain, like the kind South Africa has had much experience with in its Namibia border war. A dull, black 155-mm howitzer mounted on top can fire accurately at a distance of 24 miles.
Defense Minister Magnus Malan said, ''We hope it will find a place in the arsenal of friendly countries.''
South Africa's stated policy is not to sell arms to communist nations or countries that are in any way engaged in a war effort against the republic. Armscor officials have indicated they see the strongest market for their arms in South America, the Middle East, the Far East, and Africa.
Armscor chairman Commandant Piet Marais said South Africa exported arms worth about $9 million in 1981 but was now engaged in much more ''aggressive marketing'' to push the sales figure higher.
According to the United States Arms Control and Disarmament Agency, South Africa was ranked No. 24 in arms exports in 1979, the latest year for which data is available. South Africa equaled Brazil and Spain in exporting some $60 million worth of arms in that year.
South Africa, say most analysts, has become nearly self-sufficient militarily , despite the United Nations arms embargo of 1977. The Army is the most self-reliant branch of the military and its armored cars and mine-resistant vehicles are considered to have good export potential.
South Africa has achieved least self-sufficiency in its Navy and Air Force. If the arms embargo were lifted, analysts say Pretoria would probably be eager to buy helicopters, supersonic fighters, and long-range reconnaissance aircraft.
In areas where South Africa does not have its own production, it gets by trying to maintain equipment bought before the embargo and with high-price purchases on the black market, say analysts. It is considered weakest in advanced electronics.
''They are over the hump in self-sufficiency,'' says one diplomatic source. ''They would still love to see the embargo lifted, but gradually over the next few years this will be less and less important to them,'' he reckons.
Still, the embargo has clearly forced South Africa to invest huge sums in arms production - investments that are apparently proving painful to sustain without the assistance of more sales revenues.