Railgun

"Fire forward railguns at the lead ship."

- Paul Emerson

Railguns are weapons built by the Tau'ri, and are used on all Earth-built ships, and are one of the more impressive Tau'ri original inventions.

Overview
"That is a big gun."

- Eli Wallace

Railguns are high-velocity projectile weapons which use electromagnetic fields to propel a solid metal slug at a velocity of mach 5 over a distance of 250 miles. A standard magazine carries ten thousand rounds. This impressive ammunition capacity was the result of an upgrade deemed necessary after the Prometheus used up nearly all of its ammunition in a relatively small engagement. They have become standard on F-302s, the Prometheus, and all Daedalus-class vessels for ship-to-ship combat. They are mainly used as point defense weapons, as larger capital ships, particularly shielded ones, are much more resistant to them.

A railgun can easily shoot down fighters such Wraith darts and Death Gliders in a few shots. Larger or shielded vessels, such as Ori warships, Goa'uld Ha'taks or Wraith Hive Ships, are virtually immune as long as they are expecting the attack. During the Battle of Asuras, an opening salvo of railguns proved powerful enough to cause severe damage to an Asuran-built Aurora-class warship, in tandem with several Wraith vessels bombarding the vessel. However, the ship likely had its shields down at the time as it wasn't expecting the attack, since the weapon by its very nature is more effective against armor than shielding.

In general, railguns have been shown to be insufficient in defending any target by themselves. Railguns installed in Atlantis during the siege were only successful in holding off large waves of darts when supplemented with dozens of drones, and they only moderately succeeded in protecting Icarus base when it came under attack, though at both times, they were human-controlled. Despite advantages of surprise and superior shielding technology, the railguns on the Daedalus repeatedly proved insufficient to penetrate the organic armour on the larger Wraith ships, like the Hives and Cruisers, and only prove useful in situations where ships with powerful shields, and thus little need for armor, were caught with them deactivated, or when they are used against smaller ships, like Darts or Death Gliders.

Recently, Asgard plasma beam weapons have replaced railguns as the primary armament on Daedalus-class battlecrusiers, and railguns seem to be used as a point defense system for small fighters, a much more suitable task given fighters rarely posses shields.

Behind the scenes
In reality, the technology exists to build railguns; however, it is difficult to obtain sufficient power for them to be effective and to get the barrel of the gun in question to withstand the extreme temperatures and speeds of the projectile, wearing out the barrel with only a few uses. Presumably, the fictional explanation for the ability to make operational railguns would be the invention of naquadah generators and the discovery of more durable materials, such as trinium. In Stargate, the apparent Gatling gun-style configuration may help to keep the barrel from overheating. In addition, real-life railguns sound very different from those portrayed in Stargate (not to mention that there is no sound in space) - while the Stargate railguns make a high-pitched firing sound, a real railgun produces a deafening, low-pitched boom when firing (a result of superheated gases expanding violently as the projectile leaves the turret).

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