Sydney's 10mm "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.
10mm SMG - The standard version found throughout the Fallout 3 game world.
Sim versionOperation: Anchorage (add-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.
10mm Alloy Steel Submachinegun - Another version of the weapon was cut from the Operation: Anchorage DLC. Called the 10mm Alloy Steel Submachinegun, it is otherwise identical to the standard 10mm SMG.
End of information based on Fallout 3 cut content.
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 "death item". 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.
xbox360ps3pc It can be obtained an infinite number 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 number of times. This was fixed as of patch 1.7. [verified]
xbox360ps3 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. This continues to happen even in the Game of the Year version of the game. [verified]