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 incredibly powerful weapon in close combat, the SMG can eat through enemies extremely fast thanks to its high fire rate. Its efficiency rapidly decreases with range, as without Concentrated Fire it is too inaccurate. However, it remains a powerful weapon in confined spaces, particularly against particularly durable enemies, such as mirelurks and mirelurk kings.
Note that it requires a large ammo pool due to its fire rate. It also degenerates fairly rapidly and can fire 1186 rounds, or about 40 magazines, before breaking. However, it's common enough to make repairing it easy.
Operation: Anchorage The sim version from Operation: Anchorage is a variant with 999,250 hit points, making it invulnerable. It is not available outside the simulation. A separate version, called the 10mm alloy steel submachinegun is included in the game's files, indicating that the weapon was supposed to have an unique skin and be available outside the simulation.
The Lone Wanderer kills a super mutant with a 10 mm SMG.
↑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.