I could be wrong as far as the actual happening goes, but in the movie, the Martini was the rifle that was borrowed, and missfired when the lion was in it's sights!

In the book, I don't remember mention of a Martini, but I wouldn't doubt there was on in camp, because those old Black powder 577/450 Martinis were very popular rifles!

The double rifle in that movie, looks exactly like a little Westley Richards rifle I own, and apears to have the black powder long 28" barrels of that period, Mine is a 500/450#1 Express, and black powder rifle made in 1892, hammerless ejector rifle! It , in fact, was most likely a stage prop, and not a real rifle at all.