That's true Paul, although the longer barrel version would have been the firearm used at Rourke's Drift. As you say, the shorter barrel Martini Henry carbine was principally used by the cavalry. Either way, the bore was .5" and used a giant (by todays standards) round which would have stopped an elephant.
Just to illustrate how times have changed, in 1960 whilst at school, I swapped a Crimea vintage cavalry sabre and a WWII vintage Japanese Samuri sword for 2 Rourke's Drift vintage British Army Martini Henry single shot rifles, 1 infantry and 1 cavalry, and rode home with them strapped to the crossbar of my bike. Needless to say, no-one thought that was in any way remarkable, and my parents didn't even make any comment.
I cannot for the life of me remember what happened to them though. I probably swapped them for some green marbles and a ball of string....