Barrel

Barrels were used to safely contain Krillitane oil by the Krillitanes in Deffry Vale High School. One of these barrels were struck by K9 Mark III's photon beam, resulting in the school exploding with K9 and the Krillitanes inside. (TV: School Reunion)

The phrase "like shooting fish in a barrel" referred to doing something that was really easy. The Fourth Doctor compared tricking the guards on Zanak to shooting fish in a barrel. (TV: The Pirate Planet) Henry McMullan compared shooting the Drelleran to fish in a barrel. (AUDIO: Destination: Nerva)

Behind the scenes
Barrels with highly explosive contents were kept in the Vault, proving a safety hazard. Security personnel were advised against discharging firearms nearby, as exploding barrels were known to leave holes in floors and nearby people. These barrels were easily destroyed by the Dalek's gunstick. (NOTVALID: The Last Dalek)