The video: After six years of development and at least $240 million, the U.S. Navy's futuristic electromagnetic railgun is one big step closer to reality. The Office of Naval Research (ONR) is now test-firing a working prototype of the weapon that's small enough to fit on a warship. Using electric pulses, not chemical explosives, the cannon can shoot a 40-pound metal slug from New York to Philadelphia at up to 5,600 miles per hour — more than seven times the speed of sound — with 32 times the force of a car traveling at 100 miles per hour. (Watch a video below.) "This is the stuff you saw in movies a couple of years ago — cutting-edge, taking out the Transformers — and now it's reality," says ONR chief Adm. Matthew Klunder

The reaction: Meet "the ultimate superweapon," says David Woods at Manolith. The railgun is already deadly with these test projectiles — basically non-aerodynamic hunks of metal — and once the railgun starts using real ammo, "it could revolutionize the way battles are fought." But watching this "railgun in action chills me to the bone," says Jacqueline Burt at The Stir. "I'm not disputing the incredible technical achievement this signifies," but I could say the same of the atomic bomb. Can any good "ever come from creating a weapon of mass destruction"? See for yourself: