Heat guns were a type of energy weapon developed by the Thains and used in the 29th century. They stopped using them when they began living in peace, and a century later they were found only in museums.
When the Kleptons attacked a Thain city, they were unable to defend themselves. The First Doctor, John and Gillian encouraged them to acquire heat guns from their museum and use them to shoot down their flying machines and kill one of their creepers. When the time travellers followed the Kleptons to their underwater city, John took a hand-held heat gun with him, and used it to blast their way out of a cell after they were captured. (COMIC: The Klepton Parasites)