This is a weapon in Perfect Dark. A big fish-like blobby cyan machine-gun, it is the default machine gun carried by Maian soldiers. "Callisto" is the name of a moon orbiting Jupiter, which in turn takes its name from a character in Greek mythology. "NTG" may or may not stand for "Neon Turquoise Gun". Like all Maian weapons, the Callisto reloads in a liquidy sort of way, and does so pretty slowly - it's quicker to switch weapons twice than wait for it to reload.
You get to use it on Ruins. It can also be obtained on Attack Ship and War, but not if you're pursuing a speed strategy. It's mainly used for taking down the Skedar King. You should ALWAYS use the secondary function instead of rapid fire, as this cuts down hugely on reloads.
The Callisto NTG is also the subject of three challenges in the Firing Range.
|Perfect Dark Firing Range|
Pistols: Falcon 2 / silenced / scoped | Magsec 4 | DY357 Magnum / LX | Mauler | Phoenix