The standard assault rifle for the US Armed Forces is the M16, the Marines first created the M16A4 the newest version of the M16, before 2011 the US Army were using the M16A3 but now they are adapting the M16A4. Although the M4A1 carbine is being used more by the US Military now then the M16. In about 10 years the M4 is to be replaced.
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Those are post cold war "fads", and are equipment for the 21st century military; see US Army small arms (21st century) websites. M147.62mm Rifle - - - - - You sight-in a M2 Browning with the M2 Browning itself.
The M1 Garand was the standard issue rifle of US forces during the Second World War. However, the issue of the M1 hadn't been fully implemented at the onset of the war, so many units (particularly in the Pacific) were still equipped with the M1903, and a number of the Iver Johnson rifles were also issued throughout the Armed Forces in order to supplement the Garand.
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It's just a military nomenclature. The US uses M, presumably for "model".
After the cold war ended? The US military faced employment issues.