Fission gun

Fission guns were small but powerful hand weapons kept aboard Nerva Beacon when it was used as a cryogenic repository to preserve the human race. When the Wirrn larvae were attacking the cryogenic section, Vira ordered Rogin to fetch the fission guns from the armoury and he and Harry used them to fend off both Noah and the larvae. However, the Fourth Doctor concluded that even these powerful weapons would be useless once the Wirrn had pupated to adulthood.

Fission guns were operated similarly to conventional handguns but, when the trigger was pressed, the muzzle opened up and a crackling blue energy pulse was fired at the target. The operation of the weapon was simple enough that someone could be shown how to use it almost immediately, such as in the case of Harry Sullivan. (DW: The Ark in Space)