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.
The Ultra has been additionally customized to increase the muzzle velocity of the bullets fires. On top of that, instead of standard magazines, Sydney uses expanded magazines, giving her 50 10mm rounds per magazine.
Sydney's ultra SMG does 2 extra points of damage per shot and holds 20 more rounds of ammunition when compared against the base 10mm SMG, which also allows it to do substantially more damage per second when reloading times are considered. This puts it in the same basic place as the MP9 10mm SMG in Fallout and Fallout 2: exactly as powerful as the normal 10mm pistol compared to the normal SMG.
The Ultra SMG can fire 923 rounds, about 18 magazines, before breaking.
Sim version (Operation: Anchorageadd-on), a sim-only version of the weapon which differs only in health, with a whopping 999,250 hit points, essentially preventing the gun from ever degrading. The weapon is not accessible outside of the simulation without the use of the Gary 23 exploit or console commands.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
Sydney has two versions of this weapon. According to the GECK, she carries a non-playable version, and a playable version is spawned on her person when she is killed, as a "deathitem". As a companion, she will use up ammunition for any non-companion weapon, so the non-playable version exists to prevent this. Thus, if she dies and you take her weapon, you may notice that she appears to still be holding it in her hand.
PlayStation 3Xbox 360PC It can be obtained an infinite amount of times from Sydney if you go through the necessary dialogue choices and then give her the note from her father. After you complete the quest by returning to Abraham Washington, select the "So, whatever happened to your father?" dialogue option, then give her the note and she will give you her Sydney's 10mm "Ultra" SMG and 50 10mm bullets. Continue talking to her and you will notice the previous question about what happened to her father is gone. Select "Since we're done now, why don't you tell me about yourself?", then "Nice gun, I've never seen one like that before", followed by "That's all for now, I guess", then repeat "So whatever happened to your father?" and finally "Sydney, I found a note from your father, he didn't abandon you." This can be repeated an infinite amount of times. There is no fix for this even on the latest patch.
PlayStation 3 Sydney's 10mm SMG will sometimes flip upside down when she has it hanging on her hip, or when she draws it to walk around the National Archives.
↑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.