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 to use, making this weapon fairly cost efficient mid to late game. Although this weapon is outclassed by the H&K G11E near the end game, 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. You will need 10 AG, Fast Shot, Bonus Rate of Fire, 2 Action Boys and a P90c in each hand. You can then release a storm of 48 10 mm rounds at up to 4 targets, but you'll need to reload your guns every second turn. Even if you reload both guns, you can still fire 2 bursts and reload that gun on top, giving you another 2 fully loaded SMG's to deal massive damage. If you have the Sniper perk on top of this, you will be a maelstrom of death. Of course, you can also risk addiction and use Jet to allow you to reload one of your guns instead of waiting a full turn as well.
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 ammo. These weapons can be fired normally but can only be reloaded with the correct 10mm ammo. In addition, random encounter robbers carry 9mm ammo, 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]