Man testifies before senate committee. He has invented super powerful gun that is easily made. Can vaporize tanks and planes. Makes governments obsolete. Turns out he released plans already, committee will be obsolete.
A man uses the broadcast of a U.S. Senate hearing to describe a cheap, easily-built laser that could cut the Earth in half like a ripe tomato. He then spends several pages trying to justify distributing information that could allow any madman to destroy the planet. He later admits he had distributed the information far and wide earlier.
As pointed out by DavidW, The April 1965 issue of Galaxy Magazine, containing this story, is available at the Internet Archive.