An upgraded version of the Sniper Rifle, the Reservist's Rifle is a powerful addition to any arsenal. Finding it is a little tricky and finding replacement parts is just as tricky. But if you can keep it in working condition, the Reservist's Rifle will be one of your best friends.
Go to the Dickerson Tabernacle Chapel. It is located in the northwest section of the wastes. It is between the Deathclaw Sanctuary and WKML Broadcasting Station.
Approach the chapel carefully. Your target is a sniper called Drifter. He is sitting in an alcove near the roof. He is inaccessible by hand; you will have to shoot him out. There are usually Raiders in the area as well.
Clear the area before attempting to dislodge Drifter. The last thing you want is to get stabbed in the back by a Raider while attempting to bring Drifter down.
Kill Drifter. He can't move in the loft and as such, is quite vulnerable to explosives. A couple grenades or a single missile are usually enough to dislodge him.
Loot the Reservist's Rifle from Drifter's corpse. If he does not fall down, there are some ways to get him down. You can throw some explosives up there to blast him down or you can shoot him until a piece of him falls off, then loot the piece that falls. You can also use the console (~ key, PC version only) and engage Noclip mode, then float up to him and loot him.
There is a copy of Guns and Bullets in the chapel, usually near the pulpit. Grab it for a boost to your Small Guns skill. The Reservist's Rifle can be repaired with normal Sniper Rifles and uses .308 rounds for ammunition. Note that it only has three rounds compared to the normal Sniper Rifle's five rounds. Fast-traveling to another location, then returning can sometimes bring Drifter's corpse down from the rafters for you to loot.
Drifter sometimes suffers from a bug that will make him randomly disappear in the middle of a fight. Reload if this happens.