The beam from the weapon caused great pain and distress; those hit by it screamed as they vanished. Nyssa, taking possession of the gun, used it to destroy the Ergon.
The gun could be broken down into smaller pieces, allowing the Fifth Doctor to carry it in his pockets through the streets of Amsterdam. He reluctantly fired it at Omega to stop him from willing his own destruction, which would have caused "all" to perish. (TV: Arc of Infinity)