Part of the problem at Isandlwana was tha the ground was too hard to dig for defences and, as the Zulu were 'known' to be well away from the area, the matter was not pressed.

The ammunition box was always a red herring, the lid ware tapered and only fastened with a single screw, it could (and often was) opened by a good kick or a blow from a rifle butt (I believe a number of severly bent screws have been found at the site) - later patterns of wooden ammunition boxes were fastened by a folding 'T' shaped key.

Part of the problem with the Martini-Henry (at the time) was a tendency to jam in hot and sandy conditions - The infanrtry ammunition was made from rolled brass sheet with an iron base (made by military orphans) which, after firing were known to seperate if the body of the cartridge caught (perhaps by foreign bodies, such as sand grains, entering with the round*). The middle of an action, agains a swift enemy with a taste for 'close-quarter' combat is the last place anyone wants to be trying to remove a broken cartridge case with a jack knife.

Btw Wasn't Rorke's Drift on the same day as Insadlwana?

W


* Existing examples of the hand rolled rounds are not uniformly round and all seem to have corrugations from manufacture (expansion as the charge fired would form these to a gas-tight fit against the chamber wall. Any foreign matter in the 'corrugations' would, likewise, be driven hard against the chamber wall.
A little later, the infantry ammunition was changed to the same (and more substantial) 'deep drawn' cases used in the likes of Gatling and Gardner guns.