Rail gun

A rail gun is a weapon that employs electromagnetic induction to accelerate projectiles to an incredibly high muzzle velocity.

Operation Peace Walker
During Operation Peace Walker, a rail gun (or a device similar to a rail gun) was used by the AI weapon Chrysalis. After it was defeated, the Militaires Sans Frontieres salvaged its rail gun as well as its radome to build Metal Gear Zeke. A man-portable version (presumably an initial prototype, due to the fact that it requires a separate exterior dynamo to operate and needs at least two people to operate) was also either procured or developed by the Militaires Sans Frontieres.

1990s
In 1993, rail gun testing was presumably restarted, and had advanced.

Metal Gear REX
In 2005, Metal Gear REX was equipped with a rail gun on its right shoulder, which was originally designed to provide it with missile defense capabilities. However, it was modified without the chief designer's knowledge to enable it to launch stealth equipped nuclear weapons. By using a rail gun rather than an ICBM the weapon could be launched without using a propellant, which would be detectable. This also exploited a specific loophole that existed in anti-nuke treaties, since weapons fired from a rail gun were not technically ballistic, so limitations on ballistic weaponry did not apply. A country could therefore use REX to secretly attack any desired target without fear of retaliation through military or legal force. Solid Snake was successful in disabling REX at Shadow Moses Island, removing the danger that the Metal Gear posed. The US government then covered up the whole incident and REX was left on the island.

Fortune
Dead Cell leader Fortune's main weapon was a man-portable rail gun. It had an extremely long reload cycle making it most suitable as a sniper rifle. It also had a massive recoil effect, making it far too dangerous for normal use in combat to the enemy and to the user. Fortune was able to use it in normal combat due to her ability to deflect bullets. After her death in 2009, Vamp retrieved her rail gun and passed it on to B&amp;B member, Crying Wolf.

Guns of the Patriots
In 2014, Liquid Ocelot sent Vamp and Naomi Hunter to Shadow Moses Island to retrieve the rail gun attached to REX. The facility's stockpile of stealth "naked nukes" was the only way to directly attack JD's satellite without the Patriots knowing about it.

Crying Wolf used Fortune's rail gun in her battle with Solid Snake on Shadow Moses Island. However, Snake was able to defeat her and he ended up procuring the weapon himself. While compact, the rail gun packed as much firepower as a tank turret. It had the most penetrative power of any weapon in Snake's arsenal and dealt extremely high damage to targets. Snake later used it to hold off an entire division of Suicide Gekko.

Gameplay
The Rail Gun, which can be used in Metal Gear Solid 4, possesses 3 levels of charge. A Lv.1 charge is comparable to the DSR-1's punch, while a Lv.2 charge is comparable to explosives. A Lv.3 charge is even more powerful, allowing an instant kill on a Gekko, with one shot to the head. Ammo in-game is scarce and should be used sparingly, though more can be bought from Drebin if the player has earned enough Drebin Points.