The M-1 Garand was introduced in WWII to replace the M-03 Springfield bolt action. M1 was 8 shot.
M-14 replaced the Garand. Basically a clone of the Garand, the M-14 featured a 20 rd. mag that provided more fire power -- an issue that developed with the "wave" attacks in Korea.
Eugene Stoner introduced the AR-10 in the early 50's to compete with the M-14. The AR-10 was 7.62 cal. like the M-14. Too futuristic, and not necessary to replace the more traditional M-14.
Stoner developed the gun into a 5.56 cal and some Air Force general attended a cocktail party with Stoner where the gun was tried out on watermelons at 50, 100, and 150 yds. The General orded 80,000 for guarding air fields.
Introduced the AR-15 in early 60's in Viet Nam. MacNamara and that crew ordered them because of their comparative lighter weight.
Hard to get the traditionalists to accept the idea of a smaller caliber, less powerful round, but studies showed that most infantry were not engaging fire until under 200 yds. And the "aimed shot" was a myth.
Quote from the History Channel program:
"Only people who use an AK-47 are malcontents, political radicals, and those who can't afford an AR."