Fallout 3 -- The Pitt Cheats (PC)

The Infiltrator: This high powered Sniper Rifle can only be found in The Pitt
add-on. In the Arena, beat the final challenger and search his body for the Infiltrator. 
You may also kill the men with Infiltrators just past the gate before downtown.  Of
course, since your weapons are gone, you'll have to use the Repellant Stick from the
Wasteland Survival Guide sidequest.