DAHLGREN, Va.—A warning siren bellowed through the concrete bunker of a top-secret Naval facility where U.S. military engineers prepared to demonstrate a weapon for which there is little defense.

Officials huddled at a video screen for a first look at a deadly new supergun that can fire a 25-pound projectile through seven steel plates and leave a 5-inch hole.

The weapon is called a railgun and requires neither gunpowder nor explosive. It is powered by electromagnetic rails that accelerate a hardened projectile to staggering velocity—a battlefield meteorite with the power to one day transform military strategy, say supporters, and keep the U.S. ahead of advancing Russian and Chinese weaponry.

In conventional guns, a bullet begins losing acceleration moments after the gunpowder ignites. The railgun projectile gains more speed as it travels the length of a 32-foot barrel, exiting the muzzle at 4,500 miles an hour, or more than a mile a second.

“This is going to change the way we fight,” said U.S. Navy Adm. Mat Winter, the head of the Office of Naval Research.