High Plains Drifter

 refers to an unmarked quest in Fallout 3.

On the top floor of Dickerson Tabernacle Chapel is a sniper called Drifter. He carries the Reservist's Rifle as well as the holotape for Oasis.

He is always hostile to the player and will attack the player once they reach the building. As he is on an unreachable ledge, he must be dispatched with a ranged weapon or grenades. Sometimes he might drop from the ledge to engage the player, allowing for more options to kill him. The real challenge is to kill him in a way that puts him in a lootable position as you cannot reach the planks he is on normally.

Getting up to the Corpse

 * In the rear of the church is a soap box containing two Stimpacks and a box of laundry detergent; simply move this box directly underneath the platform. From this point, you should be able to stand on the box and jump just high enough to loot Drifter's corpse.
 * In the PC version, it is possible to reach the ledge by turning off collision and simply floating/flying to his location. (Use the console and type tcl to toggle collisions on and off).
 * It is possible, with some difficulty, to jump on the lectern below the platform and then on the diagonal planking which covers one of the windows, then turn to the left and walk slowly forwards, gaining height until the player is close enough to loot the corpse.
 * If the drifter doesn't fall to the ground you can use the upturned bench to run and jump off to reach the distance to get his gun.
 * It is possible to reach his corpse jumping from the ground just underneath it (tested on PC and Xbox 360 Version).

Bringing it down to you

 * The Gauss Rifle obtained at the end of Operation: Anchorage, as well as any explosive weapons (i.e. Missile Launcher, will easily knock him out the window.
 * Another easy method is to wait and watch him from a distance until he moves to stand in front of the window, then snipe him in the back so he falls through it.
 * Shooting at his body to dismember it after he is dead so it falls down to the ground is an option if you can't get him to fall down while killing him. Also, making his body explode, scattering pieces all around the area can help, as any body part of a corpse can be searched (e.g. eyes, legs, brain tissue, etc.).
 * If the Drifter is stuck on the upper floor of the Chapel, and you can't reach him to loot, fast-traveling to a remote location and returning may result in him miraculously falling down to loot.
 * One more way to make the Drifter's corpse accessible is by blowing up the two cars in front of the chapel. This will occasionally knock the corpse off the ledge.
 * Another option, if you have obtained the Victory Rifle, is shooting him from afar, probably resulting in a critical which will cause him to land on the ground floor.
 * The Railway Rifle is a surprisingly effective weapon to use against The Drifter. Whatever is not pinned to the wall by the spike when he is killed will most likely fall through the gaps in the boards he stands on because of the impact of the Railway Spike.
 * Another very easy way is, use a Fat Man or a Missile Launcher and shoot him. He will, 90% of the time, fall down the chapel of course, though a Fat Man seems a little gratuitous for the task.
 * Simply blow off a limb of the Drifter and search the limb.

Obtaining the rifle without killing Drifter

 * You can obtain this unique Rifle repeatedly by shooting it out his hands picking it up and running off with it. When you visit him 72 hours later, he'll be equipped with another Reservist's Rifle.(Though this is easier than many other strategies, you have to get the weapon repaired after this, as it will be damaged or broken.)
 * One successful method is targeting the Drifter's right arm, then left arm, then right arm again in V.A.T.S. Target when Drifter is reloading and rifle is held solely by right hand.  The Rifle was dropped with no damage caused by player (condition >50%).  Weapon used was .32 Pistol.  Drifter was not killed, but after doing this several times injuries will kill Drifter.  However, since the rifle isn't damaged, you won't need to do this but once or twice.

Reward

 * Reservist's Rifle
 * Oasis location