M1877 "Thunderer"

The M1877 "Thunderer" was a firearm obtained by Jacob and Evie Frye during the nineteenth century. This small model was fashioned out of silver and gold, giving it an unsubtle, flashy appearance. Due to its size, however, it could easily be concealed, allowing the Frye twins to carry it with them in the streets of London.

Reference

 * Assassin's Creed: Syndicate