Um, wow. This video comes from a test firing of the Navy’s Elecromagnetic Railgun (EMRG), which was carried out yesterday at the Naval Surface Warfare Center in Dahlgren, Virginia. The gun—which generates a powerful electromagnetic field to hurl projectiles at extremely high speeds—is rated at 32 megajoules, but the railgun engineers have to work up to that number slowly: this test was designed to reach a record-setting muzzle energy rating of 10 MJ. (The actual number turned out to be 10.64 MJ, according to Collin Babb with the Office of Naval Research.)
One big question this video begs is, what causes the giant fireball? Rail guns are supposed to be powered solely by electricity, and don't use explosives of any kind for propellant. Babb told PopSci the answer: The flames are from pieces of the projectile disintegrating; the 7-pound slug is jammed so firmly between the rails that when it’s fired, pieces shear off and ignite in the air. There’s been some speculation online that the flames come from some sort of gas that’s been used to increase conductivity. Wrong: The EMRG uses no secondary propellant — just electricity. As a result, the breech can remain open during firing and the gun produces no blowback whatsoever. In fact, the researchers sometimes place cameras and mirrors inside the breech during tests to get a better sense of what’s going on.
That flash from the projectile hitting its target is momentary, and the paper on the target isn’t burned at all afterward — just ripped and shredded from kinetic damage.
The Navy’s eventual goal is a ship-mounted railgun that can fire a projectile more than 200 miles at speeds of more than 8,000 feet per second. Context: The Navy’s current MK 45 five-inch gun has a range of just 20 miles. The Navy hopes to have a prototype ready sometime between 2016 and 2018.