The Army already knew early-on that the M14 was a non-starter when they did the classified force-on-force exercises at Fort Benning using captured Type 56s and RPDs vs M14s and M60s before any major US combat forces were deployed to Vietnam in 1965. The OPFOR Platoons equipped with 7.62x39 out-lasted the BLUE FORCES M14/M60 combo, then ran them down like dogs. They could sustain more fire on initial contact, support flanking and bounding longer, and still have plenty of ammo for overrunning/actions-on, consolidating & reorganizing, then prepping for counter-attack without need for resupply.
7.62x51 simply doesn’t support that at all. Almost everyone goes Black really fast, and direct combat support duty positions have to give up their mags to the line just to scrape by. PSG, PL, RTO, Combat Medic, FOs, etc. have to hand over their ammo and someone has to redistribute it to the Fire Teams in the Rifle Squads.
Basic load for the M14 was 5 mags, with unit discretion on augmenting as they could.
5 mags of 7.62x51 is heavier than a lot of people think, even when not carrying much else.
30rd 7.62x39 mags aren’t exactly lightweight either, but still provided a better UBL for OPFOR and real-world NVA, insurgents/VC/militia. Warsaw Pact and Chicom chest harnesses allowed carriage of more rounds for the same weight.
The AR-15 of course took round count to a totally new level of form factor, weight, low recoil, and ease of training. We still kept belt-fed 7.62x51 weapons at the Platoon-level, and then looked for a Squad-Level belt-fed Light Machinegun in the 1970s to balance out the weapons mix.
