The Heckler & Koch P90c was just coming into use at the time of the war. The weapon's bullpup layout, and compact design, make it easy to control. The durable P90c is prized for its reliability, and high firepower in a ruggedly-compact package.
The H&K P90c is a potent weapon available around mid-game. It has good damage, and costs 1 less AP to fire than all the other burst-type weapons. Because of this, it is essentially the ultimate SMG. Another major advantage of this SMG is that it puts the by-then obsolete 10mm ammo back to use, making this weapon fairly cost-efficient mid-to-late game. Although this weapon is outclassed by the H&K G11E near the endgame, the P90c is useful up to that point (even against the super mutants in the Mariposa Military Base).
Characters with certain stats can use the P90c to pop off a devastating 4 bursts per turn. This cannot be done with any other burst weapon except the M3A1 "grease gun" SMG. A player will need 10 AG, Fast Shot, Bonus Rate of Fire, 2 levels of Action Boy and a P90c in each hand. It is then possible to release a storm of 48 10mm rounds at up to 4 targets, but it'll need to reload the two guns every second turn. Even if both are reloaded, one can still fire 2 bursts and reload that gun on top, giving another 2 fully-loaded SMGs to deal massive damage. Having the Sniper perk on top of this, one will be a maelstrom of death. Of course, one can also take the risk of Jetaddiction to allow reloading one gun instead of waiting a full turn.
The actual P90 Personal Defense Weapons is produced by Fabrique Nationale de Herstal. In the game, this gun is being depicted as having been produced by another real-life gun company, Heckler & Koch. It is also shown as being either 10mm or 9mm, while the real version is chambered for FN's 5.7x28mm ammunition.
In Vault 15, some raiders have H&K P90c's incorrectly loaded with 9mm ammo rather than 10mm. These weapons can be fired normally but can only be reloaded with the correct 10mm ammo. In addition, random encounter robbers carry 9mm, but have the P90c loaded with 10mm. It seems that the P90 was originally going to be loaded with 9mm, but was changed shortly before release. [verified]