An important touch of authenticity
The director of the Pirates of the Caribbean: Gore Verbinski, was in charge of The Curse of the Black Pearl. He felt that it was important to add a touch of authenticity to the movie, which is why he insisted that Jack should use real pistols. Jack carried replicas most of the time, but during the scenes where they had to fire the pistols, they were replaced by the real ones. Two vintage pistols were crafted in 1765 by a named Perry.