The Five-seveN is a pistol designed by Belgian weapons manufacturer, FN Herstal.
The Five-seveN was developed to complement its P90 PDW (Personal Defense Weapon). Like the P90, the Five-seveN also shares the same cartridge (5.7mm x 28mm) that is designed to pierce standard body armor. Some other advantages of the pistol include it's low recoil (due to the 5.7mm round), a large 20-round magazine, and weighing in at 726 g (1.6 pounds), it's reasonably light-weight. In spite of this, the Five-seveN has not been widely adopted, primarily due to its high-cost and unusual concept. Even still, it has proven to be a successful design.
The capitalization of the "F" and the "N" in Five-seveN are references to the manufacturer of the weapon, Fabrique Nationale d'Herstal, known more commonly as FN Herstal, or simply FN.
Usage by Haven Troopers
During the Guns of the Patriots Incident, the Five-seveN was used by Liquid's personal army, the elite Haven Troopers, a.k.a. the FROGS. The weapon was the natural sidearm of the FROGS, using the same ammunition as their primary weapon, the P90.
Behind the scenes
The Five-seveN is one of the first weapons that Solid Snake finds in Metal Gear: Ghost Babel. Even though Ghost Babel is non-canon, its description in Metal Gear Solid 4: Guns of the Patriots mentions that Snake used a Five-seveN in Galuade. It also appears in Shinta Nojiri's non-canonical Metal Gear Acid titles.
The Five-seveN can be obtained in in Metal Gear Solid 4, and can be customized with an H.G. Light. It offers the highest penetration of non-Magnum pistols in-game, making it effective in some ways against a FROG. However, it is not recommended to take on a full FROG force with it, as they hunt in groups and packs heavy firepower with their P90s and will leave the player heavily outgunned.