In a BBC documentary they did a test with actual ammo,in a hot dry environment the barrel fouled quickly. Their version of black powder didn't work to well, adjustments were made after they discovered that problem.
Everything went right for the Zulus and everything went wrong for the British,the powder was just one problem.
Hmm. One try with unknown copy ammunition had a problem. More than 50 years and literally millions of cartridges fired by assorted armies and civilians and no fouled barrels. I put 35 rounds through my Martini Henry last month. No problem: I must be doing something wrong. No one has made Boxer cases since the 1930's so who knows what the BBC used? It must have come as a surprise to Turks in the Yemen, Romanians at the height of summer, Egyptians, the Indian Army in Rajasthan, thousands of hunters in Africa, Omani tribesmen and countless others to find that their Martinis all fouled beyond use within 20 rounds - apparently. Black powder rifles foul more in a hot dry environment. That is why they include a wax lubricant wad to be fired up the barrel behind the bullet. All 1879 rifles in the world were black powder. RFG2 was one of the best black powders in the world for the job of it's day. BTW Isandlwana can get bloody cold and wet during the year. Snow is known.
If black powder rifles fouling is an issue then every black powder rifle from the 17th century onwards was a 'zero' and a failure and the first military rifle that ever worked was the French Model 1886 rifle with smokeless nitro Poudre B.
We do the Zulu's a disservice to blame the kit. They out Generalled the British on the day. The Zulus won the battle through Intelligence, planning, mobility, command and control. The British lost it through incompetent command. In both cases the troops fought well, bravely and as they had been trained.