Sydney's 10mm "Ultra" SMG

The weapon used by the relic hunter Sydney. It can be looted from her by pick-pocketing, waiting for her to die, or killing her yourself.

This weapon can also be acquired by finding A Note from Little Moonbeam's Father within the ruins of the hotel in the quest Reilly's Rangers. During the mission, be sure to check the side rooms until you find a skeleton on one of the beds. You'll find an assault rifle under the bed, some ammo next to it, and the voice tape on the bed next to or under the corpse. Go back to Sydney after finding it in Rivet City and she will tell you it was from her father, that he was attempting to make one big deal and set her up for life, but was double crossed. Realizing she was wrong about her father, she will give you the gun. Note however that this can only be done AFTER returning the Declaration to Washington. After speaking to her in Rivet City, she will move to Underworld and become a weapons merchant there.

Sydney's Ultra SMG is significantly more powerful than a regular SMG (59 damage compared to 37), as well as having a much larger ammo capacity (20 extra rounds per magazine). It is also worth an additional 200 caps.

You can repair Sydney's Ultra SMG yourself with regular SMGs or duplicate copies of her gun (see bugs/exploits below).

Location
Sydney the relic hunter carries this weapon. It can be obtained by either killing her, letting her die, or finishing the quest Stealing Independence and giving her the A Note from Little Moonbeam's Father.

It can also be found by abandoning the "Stealing Independence" quest after helping her defend the area you first find her in (rotunda?). Let her run on into the dangers, and go do something else for a few hours (if not a couple days). Then come back and finish the "Stealing Independence" quest. Sydney will no longer be involved in the quest.

Later on, Sydney can also be found in various places around the Wasteland (one person found her near Raven Rock).

Bugs/Exploits
There is a bug/exploit in all versions of Fallout 3 that allows you to receive multiple copies of her Ultra SMG in perfect condition (along with 50 rounds of 10mm ammo). To trigger the exploit, after receiving the gun once choose the following dialogue options:

"Since we're done now, why don't you tell me about yourself?" "Nice gun. I've never seen one like that." (she mentions her father and re-triggers the option to ask about him)

"That's all for now, I guess" (taking you back one step in the dialogue tree)

"So, whatever happened to your father?"

and finally, "Sydney, I found a note from your father. He didn't abandon you."

You should then receive the Ultra SMG and ammo again. The above steps can be repeated as many times as you want for a limitless supply of both guns and 10mm ammo.

Sometimes, if you already completed Stealing Independence and later on found the recording, you can still show her to get her unique smg. She tells you she's heading to Underworld. However, when you get there you'll find her ammo stash in the 9th Circle bar but she won't be there. Check the Anacostia station entrance almost directly north of the Rivet City entrance. Go into the tunnels and she'll still be there, JUST starting her journey. You can escort her if you like, the walk isn't that long at all.