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 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 a 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.
 * The room is on the Statesman Hotel Mid Level. The room number appears to be 107 (but that may be the room number of every room).  If you are facing the elevator doors that take you to the Restaraunt, you are facing north, and the room you're looking for is the first to the southeast.  This room has 2 entrances.  Against the west wall are 2 bedrooms (though the wall between them is blown out).  There is a queen size bed on the left and a single/twin on the right.  The bed on the right is where you can find the skeleton, weapon, ammo, and the tape you'll need to give to Sydney.  The tape is called "A Note from Little Moonbeam's Father".

Sydney's Ultra SMG is significantly more powerful than a regular SMG (50 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).

Bugs/Exploits
There is a bug/exploit in the PC, PS3 and Xbox 360 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.