Railguns

A short article with some clickbait explains the history (railguns prototypes were first developed in World War I era France, but nothing beyond a prototype has ever been fielded for use in actual combat), and the technology of railguns (due to high energy demands and other engineering limitations they need to be quite large, suitable for artillery roles rather than small arms roles).

A railgun was planned for the DDG-1000 Zumwalt class destroyer, but this was never built due to technological challenges. 

A successor substitute for the railgun in the Zumwalt, a long range 155mm shell slug thrower with a 109km range with considerably less accuracy than existing guided missiles, called the Advanced Gun System (AGS) was also never built. The 102 kg AGS rounds would have cost $800,000 each (up from an original estimate of $35,000). So the finished destroyer, despite being the largest surface combatant in the U.S. Navy other than an aircraft or helicopter carrier, lacked the centerpiece armament it was designed around.

Other modern railgun prototypes have gone into serious testing since then, particularly with an eye towards use as an active defense to hypersonic missiles. But the last major research effort by the U.S. Department of Defense was shut down by the Navy in July of 2021:

The Navy has announced that it is pulling funds from the much-hyped electromagnetic railgun in order to shift those monetary resources to hypersonic missiles and other high-tech weapons.

The program, which began in 2005, was supposed to use magnetic fields instead of gunpowder to fire rounds at speeds of up to Mach 7 and ranges of up to 100 nautical miles. However, despite the more than 15 years that program has spent in development, it never was fielded. Navy officials continued to insist that it saw a future for the $500 million experiment as late as 2018.

“Given fiscal constraints, combat system integration challenges and the prospective technology maturation of other weapon concepts, the Navy decided to pause research and development of the Electromagnetic Railgun [EMRG] at the end of 2021,” the statement from the Navy said. The end of the railgun program was foreshadowed last month when a White House fiscal budget for 2022 revealed the Navy pulled funding for the Gun-Launched Guided Projectile -- a meter-long projectile first developed exclusively as a round for the experimental railgun. . . .

In 2018, then-chief of Naval Operations Adm. John Richardson told Congress that the weapon had yet to reach its promised range.

"That involves a number of technologies," he said. "The barrel itself is probably the limiting case, the engineering on that, the materials required to sustain that power pulse, and the heat and pressure that's involved in launching those projectiles."

Another unresolved issue was a power source for the gun. Only the Navy’s three-ship Zumwalt class destroyers reportedly were capable of supplying the electricity needed to operate the gun.

China, meanwhile, is still continuing to work on developing its own railgun. It may be in service by the year 2025.

So far, viable prototype designs need too much electrical energy, and are too large, to be suitable for use as armaments for warplanes. Railgun rounds, being basically inert metal, are themselves cheap, although the delivery system and the power plants needed to run them are not. They are better suited to artillery batteries and warships where weight is not at such a premium.

Still, despite past fails, eventually, railguns will probably become part of the standard modern military's conventional weapon arsenal.

This is because active countermeasures that work against missiles, artillery and tank shells, and drones aren't nearly so effective against railgun rounds, because railgun rounds have no guidance systems or internal explosives to target. But railgun rounds pack enough energy to overcome most armor as well. And, as noted above, railguns also have potential to serve as active countermeasures themselves.