In the early Sixties, just as the Indochina conflict was beginning to draw serious attention from the Americans, the Army noticed a gap in effectiveness between their then-new rifle, the M14, and the new Soviet AK series of rifles (commonly known in the West as the AK-47, at the time). This gap was investigated several times during this period, and one report, Rifle Evaluation Study, published by the Infantry Combat Developments Agency of the Combat Developments Command (CDC), is available for free on DTIC. This study pitted the M14 against the M14 (USAIB) – an early incarnation of what would later be called the M14E2 automatic rifle, the AK rifle, the AR-15, and the experimental SPIW The findings of this and other reports were surprising, and led directly to the adoption and eventual replacement of the M14 rifle with the AR-15 family of weapons. From the document:

5. (S) Assessment.
a. A detailed assessment of the characteristics of the
M14, M14 (USAIB), AR-15, AK-47, and the Special Purpose Individual Weapon (SPIW) to include their competing attributes and comparative merits are given at Inclosures 1 and 2.
b, The AR-15 represents a marked improvement over the M14 rife primarily because of lower weapon and ammunition weight.
Except for the SPIW, it comes closest to meeting the desired characteristics listed in paragraph 4 above. However it has two characteristics which would have to be corrected before the AR-15 were considered an acceptable military rifle in any role: its poor rifle-ammunition reliability and its poor night firing characteristics.
c. The M14 (USAIB) is a definite improvement over the
M14(M) in the automatic rifle role and in the few tests conducted has shown itself to be superior to the AR-15 in the automatic rifle role at ranges beyond 400 meters.
d. The AK-47 is basically a submachine gun and is inferior
to both the M14, (USAIB), and AR-15 in range effectiveness,
ammunition lethality and other desired rifle characteristics.
e. The SPIW, if and when developed to design criteria,
will come closest to meeting the desired characteristics.

Despite the AR-15’s good reliability in earlier tests (in at least one case, better than the M14), by 1962 it has begun to show a few warts. Even so, the merit of the small-caliber high velocity assault rifle can already be seen, with the AR-15 besting not only the M14 for effectiveness, but also the AK rifle, which the examiner considered to be but a lowly submachine gun; less effective and shorter ranged than the other rifles tested.

Many other similar rifle evaluations were also performed around the same time, with similar results. It had become clear that the AR-15 and its small caliber ammunition represented a step forward, albeit one that needed a little more work before it was ready for prime time.