Factual error: Sir John claims to have removed the gunpowder from the silver bullet four bore cartridges. However, he had not removed the primers, thus there would not have been a mere click from the falling hammers but a most definite bang from the primers. Furthermore, in some calibers the pressure from the primers may actually be enough to propel the bullets even without gunpowder.
Factual error: The film is set in 1891 and shows London's Tower Bridge in operation. The bridge wasn't opened until 1894.