The railgun faces many technical barriers before it is battle ready. Policy makers also must weigh geopolitical questions. China and Russia see the railgun and other advances in U.S. missile defense as upending the world’s balance of power because it negates their own missile arsenals.The railgun’s prospective military advantage has made the developing technology a priority of hackers in China and Russia, officials said.Chinese hackers in particular have tried to penetrate the computer systems of the Pentagon and its defense contractors to probe railgun secrets, U.S. defense officials said. Pentagon officials declined to discuss the matter further.The Navy began working on the railgun a decade ago and has spent more than half a billion dollars. The Pentagon’s Strategic Capabilities office is investing another $800 million—the largest share for any project—to develop the weapon’s defensive ability, as well as to adapt existing guns to fire the railgun’s high-tech projectiles.