The movie is irrelevant being very fictional though entertaining. It would be far more entertaining without Michael Douglas's boorish un-necessary character.

Considering Patterson used a 12-bore sidebybside shotgun and acLee Speed or similar .303, and borrowed a single shot it's really comppletely academic and irrelevant what a modern rifle maker chambers a Mauser 98 Rigby "Big Game" "Tsavo" model rifle in. Patterson didn't use a scope either ...

Wisely choosing a modern very desirable cartridge chambering which will sell well for actual safari hunting.

PS I did a search on "Man-eaters of Tsavo", "Ghost and Darkness" is not mentioned even once.