One excellent reference sits at
https://en.wikipedia.org/wiki/Martini%E2%80%93Henry covering the technical and practical data of the Martini Henry.
Another reference covers the battle at
https://en.wikipedia.org/wiki/Battle_of_Isandlwana
One of the darkest days for the British Army was the battle of Isandlwana against the Zulu. Much of the blame was initially rumoured upon malfunctions or getting the MH ammunition boxes opened. This was untrue and although a number of weapons went unserviceable due to soft case ammunition separating in the breach; this would have been manageable under normal battle conditions.
Essentially the sacred maxim of absolute solid all round defense at all halts was disobeyed at their peril and in particular an uncontrolled mish mash of troops all over the battle field surprised by a huge fast moving tribal infantry ...which essentially over ran the British before they were able to regroup. All they needed to do was form a defensive square and laager up the wagons in it perimeter. Such was the lethality of the Martini Henry that such action would probably have saved the day...
Later some distance away at Rourkes Drift another group of 140 British armed with the same weapon were to beat off 4,000 Zulu by being organised into just that...a solid all round organised and well led defence... if not a little Welsh singing by the Sergeant Major...