A disintegrator gun was a handheld weapon which effectively "disintegrated" the target.
In an alternate Earth timeline in the 22nd century, one such device employed an ultrasonic beam. In various forms, it was the weapon of choice of the human resistance opposing the Daleks and the Ogrons of that time. They were apparently made of steel composites. (TV: Day of the Daleks)
Components for another disintegrator gun were stolen in the 20th century by the K1 robot, on orders from the Scientific Reform Society. It was used to break into Joseph Chambers' safe, which contained the destructor codes for the world's nuclear missiles. The Brigadier retrieved it and fired it upon the robot. Its living-metal status allowed it to absorb the power and use it to grow. (TV: Robot)