The Mezon Disintegrator 7.5 was a thin, tubular, hand-held weapon. It was not intended for human use, having dials designed for a species with smaller digits. The known span setting was 13-85 to 13-89, with killing an individual being 13-85 and destroying a city being 13-89.
Veec-9 had a Mezon disintegrator 7.5. When the Tenth Doctor seemed to be helping Darac-7, he planned to use the gun on the Doctor. It was later fried by the Doctor when they made peace. (PROSE: Ghosts of India)