This weapon was probably cut from the game early on, when the developers realized how useless a single-shot flintlock-weapon would be; Anne is apparently incapable of changing the magazine in a basic pistol, so a muzzle-loading (or breech-loading) weapon wouldn’t be very practical. Even if she could, the time it would take to reload while running from a pursuing dinosaur would make the attempt pointless.
“Flintlock” is the general term for any firearm that uses a flintlock mechanism, or for the mechanism itself. It replaced many earlier forms of firearm-ignition techniques when it was first introduced in the early 17th century. It continued to be in common use for nearly two centuries afterwards, until it was replaced by the percussion-cap, and eventually cartridge-based systems.
Although it is far superseded by modern firearms, flintlock weapons continue to be used by black powder-enthusiasts to this day.