The SMG was introduced in Marathon Infinity and did not feature in any earlier games. It is a highly accurate mid- to long-range weapon, but due to the small magazine size and minimum of 2 rounds fired at a time, the user is forced to reload the KKV-7 quite regularly.
The flechette gun is also the only weapon capable of firing underwater without injury to the player, as well as in a vacuum.
The KKV-7 is first obtainable in Marathon Infinity, hidden behind a secret door on the level Poor Yorick.
"...the flechette, with its small cross-section (4mm), longer body and pointed nose experiences far less wind resistance and thus retains more muzzle velocity over a greater distance with a flatter trajectory that its less advanced "slug-thrower" counterparts. Its armor piercing capabilities rival those of the mighty M1A2 .75 BR and the bio-organic trauma it inflicts is terrifying (being somewhat less effective against mechanical targets)."
- from Marathon Infinity manual