[UPDATED April 8 with more rail gun & laser detail from Rear Adm. Klunder]

NATIONAL HARBOUR: 23 pounds ain’t heavy. But it sure hurts when it hits you going at seven times the speed of sound.

That’s what a prototype Navy weapon called a “rail gun” can do, and it does it without a single gram of gunpowder or rocket fuel — just electricity. For many missions, a rail gun is better not just than current cannon but than the laser weapons the Navy is testing this summer in the Persian Gulf (I’ll explain why in a minute). And, after years in development and hundreds of test shots on land — see the video for a small sample of the destruction — the rail gun is finally going to go to sea.

“We’re beyond lab coats, we’re into engineering now,” Adm. Jonathan Greenert, the Chief of Naval Operations, told the audience at the Navy League’s annual megaconference here, Sea-Air-Space 2014. “It’s going on a Joint High Speed Vessel in 2016.”

Just in time for the Navy’s biggest gathering of the year, the Sea-Air-Space conference, the Navy released this video and issued new details of the test plan. Both rail gun prototypes will be shown off to the public in San Diego this summer, aboard the new Joint High Speed Vessel USNS Millinocket. Then the Navy will install either the BAE Systems prototype or the General Atomics one — that hasn’t been decided — on Millinocket for at-sea test shots in 2016.

It’s a crawl-walk-run approach, however. The 2016 tests will only involve one shot at a time. Firing multiple rounds in a row will wait for another series of tests in 2018. Actually installing a rail gun permanently on a combat ship — Millinocket is a transport with a civilian crew — is even further in the future. Meanwhile, while one prototype or the other is doing the tests at sea, BAE is already working on a “Phase II” rail gun with such improvements as an automatic multi-loader for rapid fire and better heat control so rapid fire doesn’t melt the barrel. (General Atomics didn’t win a Phase II contract).

Meanwhile, Pentagon officials have been impressed with the Navy’s tests and are exploring the idea of a land-based version of the rail gun for missile defense, a mission currently performed by expensive and often unreliable anti-missile missiles.

So why do rail guns matter, besides generating cool clickable video? Three words: impact, range, and reloads.

Impact. Accelerated electromagnetically down a set of rails — hence the name — that 23 pound projectile moving at Mach 7 has 32 megajoules of energy. The Chief of Naval Research Rear Adm. Matthew Klunder, likened the impact to “a freight train going through the wall at a hundred miles an hour” in a recent phone call with reporters. It doesn’t have an explosive warhead, but then it hardly needs one. According to official Pentagon modeling, the sheer impact is enough to meet “every single mission” the Navy and Marine Corps have for naval gunfire, although some really tough targets may require multiple shots. With the right targeting system, the rail gun could shoot down incoming aircraft, cruse missiles, and even ballistic missiles. Lasers can do the anti-missile mission too, but they probably won’t have power for harder targets for many years to come.

[UPDATED: The Navy’s working on a laser with five times the power of the one headed for field-testing in the Gulf, “[but] today, it’s more defensive in nature,” Klunder told me when I caught him after his public remarks at Sea-Air-Space on Tuesday: Since lasers fire at the speed of light and keep firing as long as they have electrical power, they’re well-suited at defeating waves of incoming enemy missiles or drones that would exhaust the ammo supply of current defensive systems, but they lack a rail-gun’s long-range punch.]

Range.The rail gun can hit targets “over a hundred miles” away, said Klunder. That’s farther than existing Naval guns and even the Navy’s standard anti-ship missile, the aging Harpoon. That’s farther than the 65 nautical mile minimum distance the Navy calculates its ships must stay away from shore to stay (mostly) out of range of land-based missiles.

Historically, a Marine Corps landing force goes ashore from ships drawn up just five miles offshore. In the future, Commandant Gen. James Amos said at Sea-Air-Space this morning, “it may well find itself sitting out a hundred-plus miles.”

A rail gun also shoots farther than lasers, because of simple physics: Even the most powerful laser will fire a straight line-of-sight shot that eventually goes off into space, while a rail gun can fire a solid shot in a ballistic trajectory against targets beyond the horizon. On the other hand, 100-plus miles is a fraction of the range of the Tomahawk cruise missile, the Navy’s standard weapon for hitting targets on land. We used to have an anti-ship Tomahawk but made the mistake of phasing it out in the 1990s, leaving a big gap in the Navy armory that megacontractor Lockheed Martin is now developing a new missile to fill. So the rail gun is not going to be the one-size-fits-all weapon of the future, just an important part of a mixed arsenal of complementary weapons.

[Updated: “There’s ever, never a single golden BB or a silver bullet,” Klunder told me Tuesday. “We may find that the future of your battleforce may indeed be a rail gun that gives greater distances” — for both offense and defense — “and maybe a laser system that gives you more mid- to close-in range [defense],” with missiles for specialized missions such as hitting targets beyond rail gun range. Imagine concentric circles around a US Navy ship: an inner ring covered by lasers, a middle ring by rail guns, and an outer ring by cruise missiles. But the Navy hopes to use a lot fewer missiles in the future.]

Reloads. Missiles are bulky and expensive, with price tags in the hundreds of thousands or millions of dollars. A 23-pound slug of metal is, by comparison, shockingly light and cheap: about $25,000 per shot, according to Klunder. [Updated: Klunder said Tuesday at Sea-Air-Space that a laser shot is even cheaper — 59 cents per zap — but the laser does a different mission]. The Navy’s existing DDG-51 and DDG-1000 destroyers can carry about 80 to 96 major missiles in their Vertical Launch Systems. With the rail gun, the Navy can fit — and afford — “hundreds” of rounds per ship, said Klunder. That means a rail gun ship can hit more targets, from incoming missiles to enemy ships to bunkers deep inland, and it can stay in the fight longer.

What that future rail gun warship will be is an open question. JHSVs are transports: The Navy is using them as the testbed because they have a nice wide flight deck to stick the gun on and lots of cargo space to carry the electrical power and other systems. Interestingly,  one variant of the Navy’s controversial Littoral Combat Ship, the USS Independence class, is the JHSV’s big brother and has similar characteristics, since it’s designed with a huge flight deck and spare room for plug-and-play “modules” of equipment for different missions. LCS is also under fire for not having enough firepower, a major factor in an ongoing Pentagon review of whether the program should continue.

It’d be harder to retrofit the rail gun on an existing destroyer — a larger ship than LCS isn’t designed for plug-and-play — but it’s doable. In the long run, however, making full use of rail guns probably will require a new class of ship, one with much more electrical power. That’s a goal that will take even longer and even more money than the rail gun itself.

Updated Tuesday 1:45 pm with more information from Rear Adm. Klunder.