The US Navy is planning sea trials for a weapon that can fire projectiles at seven times the speed of sound using electromagnetic energy.

Chief of Naval Research Rear Admiral Matthew Klunder told a roundtable group recently, the futuristic electromagnetic railgun had already undergone extensive testing on land and would be mounted on the USNS Millinocket, a high-speed vessel, for sea trials beginning in 2016.

"It's now reality and it's not science fiction. It's actually real. You can look at it. It's firing," Rear Admiral Klunder said.

Railguns use electromagnetic energy known as the Lorenz Force to launch a projectile between two conductive rails.

The high-power electric pulse generates a magnetic field to fire the projectile with very little recoil, officials said.

Rear Admiral Klunder planned to discuss progress on the system with military and industry leaders at a major maritime event, the Sea-Air-Space Exposition, near Washington.

"It will help us in air defence, it will help us in cruise missile defence, it will help us in ballistic missile defence," he said.

"We're also talking about a gun that's going to shoot a projectile that's about one one-hundredth of the cost of an existing missile system today."

The railgun can fire a low-cost, 10-kilogram projectile at 8,575 kilometres per hour.

The Navy research chief said that cost differential - $25,000 for a railgun projectile, versus $500,000 to $1.5 million for a missile - will make potential enemies think twice about the economic viability of engaging US forces.

"That ... will give our adversaries a huge moment of pause to go, 'do I even want to go engage a naval ship?'" Read Admiral Klunder told reporters.

"You could throw anything at us, frankly, and the fact that we now can shoot a number of these rounds at a very affordable cost, it's my opinion that they don't win."

US officials have voiced concerns that tight defence budgets could cause the Pentagon to lose its technological edge over China, Russia and other rivals, who have been developing anti-ship ballistic missile systems and integrated air defences capable of challenging US air and naval dominance.

Weapons like the electromagnetic railgun could help US forces retain their edge and give them an asymmetric advantage over rivals, making it too expensive to use missiles to attack US warships because of the cheap way to defeat them.

The US Navy has funded two single-shot railgun prototypes, one by privately held General Atomics and the other by BAE Systems.

Rear Admiral Klunder said he had selected BAE for the second phase of the project, which will look at developing a system capable of firing multiple shots in succession.

"We're talking about a projectile that we're going to send well over 100 miles (160 km), we're talking about a projectile that can go over Mach 7 (8,575 kph), we're talking about a projectile that can go well into the atmosphere," Rear Admiral Klunder said.

Ships can carry dozens of missiles, but they could be loaded with hundreds of railgun projectiles, he said.

"Your magazine never runs out, you just keep shooting, and that's compelling," Rear Admiral Klunder said.

The 2016 sea trials will be conducted aboard the joint forces high-speed cargo ship because it has the space to carry the system on its deck and in its cargo bay.

Officials said they would begin looking at integrating the system into warships after 2018.

Reuters