Commonly considered to be one of the finest weapons in the world, the Heckler & Koch MP9 submachine gun fires the powerful 10mm ammunition. Although the single shot is comparable to the Colt 6520 or the N99 pistol, the MP9 truly shines in burst and fully automatic modes. Coupled with the compact design, sturdiness, and light weight, it becomes a powerful asset in the hands of anyone in the wasteland. Although, the high fire rate means the urge to spray-and-pray must be controlled, unless the operator wants to run out of ammunition. Thankfully, the ease of handling makes this easier than it seems. An unique feature of the weapon is its two selectors: one for switching between safety, single shot mode, and automatic mode, and another for specifying the size of the burst, up to fully automatic fire.
An excellent automatic weapon that will replace the 10mm pistol immediately upon finding it. The burst mode is perfect in close quarters and particularly point-blank range, finding use for the 10mmammunition that would otherwise remain unused once shotguns and rifles come into play.
Ian and Katja can use the submachine gun, but they tend to use the burst mode a lot and will often hit the player. Ian in particular has developed a nasty reputation for this.
Vault 15: Inside the rusted locker on the command center level.
↑Vault Dweller's Survival Guide p.5-16: "One of the finest weapons in the world. The H&K MP9 is a solid submachine gun, capable of single or burst mode attacks. The single shot is acceptable, comparable to the 6520 pistol, but the burst mode is spectacular! The MP9 is easy to control, and spews 10mm death like no other firearm in it’s size category."
↑Models are presented together as per the precedent established by the CZ57 Avenger in Fallout: New Vegas; since the two submachine guns look very much alike, have the same niche, and perform the same function, they are most likely intended to be the same model.