M199 assault rifle

Perhaps the most well-recognized weapon in the Pre-War U.S. Military arsenal, the M199 earned its nickname "The Backtalker" from the unusual report the firing mechanism makes as it ejects a spent shell. Firing standard 5.56mm rounds, the M199 was the official infantry weapon of the U.S. Army.

Appearances
The M199 doesn't actually appear in any game, and is mentioned in the Museum of Technology in Fallout 3. In the game itself, the default assault rifle model is the R91 Assault Rifle, a National Guard weapon.

Inconsistency
The description of the R91 Assault Rifle in a Citadel terminal states that the M series was discontinued in favor of the R series in the early 21st century, which contradicts the information in the Museum of Technology about the M199 still being the official US Army weapon (stated in present tense). It is a possibility that the museum's exhibit was in place before the military made the decision.