The misfire rifle was a Westley Richards. It was in Patterson's book as well as the movie.
I really loved the movie. And, because there is no way you can factually go back and say what exactly happened, it was made as a "fictitious" movie. In fact, though, each of the incidents that happened were true. The numbers of victims vary by different accounts. But, by most accounts, (people who consider Indians, and native Africans as humans) it was over 100.
As was common at the time unfortunately, more people died on that project from disease than from the lions. But, that made it no less terrifying in how they were able to kill so many workers... and Bhwana's, in such a short time.
The method and boldness of attacks by these lions was stunning. Patterson's book devotes a little over half of it to the lions and the hunt to kill them. He devotes the rest to the other incidents of note that he encounters, like the red cobra that came to visit him at his tent.