The R Series of combat assault rifles was used reliably following the discontinuation of the M Series in the early 21st century. The R series, produced by civilian manufacturer Stent Security Solutions, was made in many models and variations. The variation most common to Washington, D.C., was the R91 full-auto urban assault rifle. The R91 assault rifle was issued in the United States Armed Forces during the urban pacification operations and combat.
Since the National Guard had been fully deployed around the DC area during the time of the Great War, the R91 is readily available for those willing to search around this area. Although nearly two centuries of scavenging has reduced the numbers of working units, the R91 remains a favorite of slavers, raiders, and mercenaries in the Capital Wasteland. 
The R-91 is a general-purpose automatic firearm, effective from short to medium range. However, the high spread and low damage per shot greatly reduce its effectiveness against targets at great range, or hard targets like giant radscorpions. Nevertheless, it remains a lightweight, hard-hitting weapon for low to midlevel Small Guns users.
Condition degrades at a moderate rate. Poor-condition assault rifles are sold by many vendors across the Wasteland, and are found in raider-occupied camps, buildings, and Metro tunnels.
The basic assault rifle differs from the Chinese assault rifle: the latter does more damage (88 DPS compared to the R91 assault rifle's 64 DPS), but is also scarce and less durable. Spread and magazine capacity are identical.
The assault rifle can fire a total of about 1250 rounds, the equivalent of 53 reloads, from full condition before breaking.
InfiltratorThe Pitt (add-on) - A scoped, silenced, and stockless version of the R-91 assault rifle, it functions similarly to the silenced 10mm pistol in that, while not as loud, it deals less damage than its roaring cousin.
PerforatorThe Pitt (add-on) - A unique version of the Infiltrator, it has the slowest rate of fire of any assault weapon.
Sim version Operation: Anchorage (add-on) - A sim-only version of the weapon that differs only in health (essentially non-degrading). It features a cleaner, less-battered texture with fresh wood and darker metal areas than its wasteland counterpart.
The following is based on Fallout 3 cut content and has not been confirmed by canon sources.
Wanda - A unique R-91 assault rifle that was cut from the final game.
Alloy steel assault rifle Operation: Anchorage (add-on)- A variant named "alloy steel assault rifle" can be found in the data files for Operation: Anchorage. It uses the Anchorage-specific texture for the weapon, but aside from name and appearance is identical to the standard R-91 assault rifle. It is likely the weapon was intended to be placed in the armory with other "wasteland" versions of weapons found in the Anchorage sim, most of which were cut from the final version of the add-on; as such the weapon can only be accessed through the use of console commands.
End of information based on Fallout 3 cut content.
Raiders carry them very often, mostly at mid-levels.
Inside Evergreen Mills. The raiders that occupy the inside of the building use higher level weapons than the raiders on the outside, as they mainly utilize the N99 10mm pistol.
There is also a truck and barricade with some ammunition as well as a 10mm pistol in addition to the R91 to the slight northwest of Fairfax. It is guarded by a Mister Gutsy. The rifle is in poor condition when found.
There is 1 assault rifle at level 6 (confirmed) in Hamilton's hideaway with Three Dog's weapon cache key.
xbox360ps3pc The assault rifle can clip through targets or level geometry in V.A.T.S. This results in all shots missing or being absorbed by geometry, even when the muzzle protrudes past the obstruction. Against point-blank targets in V.A.T.S., the rifle sometimes inflicts minimal damage.