P90c

A P90 or P90c is a name given to sub-compact submachine guns in the Fallout universe. They boast high damage, low weight, and a helicial feed system that allow them to carry a startling amount of ammunition. While they have an odd appearance, they are the favourite submachine guns of many late-game factions.

H&K


A P90 produced by Heckler & Koch. It's chambered for the large and powerful 10mm round, and is the be-all end-all of the 10mm weapon progression line.

FN


A P90 produced by Fabrique Nationale. It's chambered for the lightweight and common 9mm round, and serves as one of the best submachine guns throughout the game. The P90 is only eclipsed by the Ruger AC556F.

Behind the scenes
The real-world P90 is produced by FN, and doesn't have a 'c' designation in the line. They're also chambered for the new 5.7x28mm armor-piercing round, instead of 9 or 10mm. Another difference is their magazine; where the game states they have a Helical feed, in reality they have a horizontally mounted feed with the magazine fitting flush with the top of receiver, parallel to the barrel.