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 horizontal 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.

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 FN 5.7x28mm armor-piercing round instead of 9 or 10mm.