M Series

The M series of weapons was discontinued in favor of the R series in the early 21st century.

Background
The M series was discontinued in according to the description of the R91 assault rifle in a Citadel terminal in Fallout 3. This contradicts the information in the Museum of Technology about the M199 still being the official U.S. Army weapon (stated in present tense). It is a possibility that the museum's exhibit was in place before the military made the decision.