Aware that, with a dinosaur in capture, the Doctor would be able to find his base, Whitaker told Mike Yates to fix a device he handed him to the gun, which would render it useless. He did so and, when the Doctor attempted to use it on an apatosaurus, it had no effect.
Whitaker used a time eddy to produce a Tyrannosaurus Rex nearby in a bid to kill the Doctor. Unwilling to let him die, Yates picked up the gun, removed the device he had attached and rendered the creature senseless. (TV: Invasion of the Dinosaurs)