Why was the M14 so bad then? Even beyond the problems with the original build quality.
M-14 was/is actually an excellent weapon, especially in the hands of a well trained rifleman. In WW II or Korea with would have kicked ass. Had the Red Army tried the Fulda Gap it would have been the right weapon. The complaints about it being unstable on full auto are sort of silly, good luck controlling any rifle as you burn through 20 rounds. Best you can do is start firing low and keep the barrel pointing at the target as the the muzzle walks the rounds up (hence the comment about well trained rifleman; they can snap off 2-3 rounds, even in a full auto weapon, diring the 2003 Iraq War you could always tell when the U.S. or UK troops were firing: "bip-bip or bip-bip-bit" Iraqi Army AQAP "rat-at-at-at-at-at-at..." spray & pray until the bolt locked open)
Unfortunately a 9.2 pound wooden stock battle rifle with a loaded magazine weighing in at 1.5 pounds each wasn't the right weapon for Vietnam where you had LURPing and even basic patrols out of contact for a couple days at a time in the jungle. M-16 came in under 6.5 pounds, with each mag coming in at 1.2 pounds. An M-16 and 60 rounds weighs less than an M-14 and 20 rounds. With a standard load of 120 rounds an M-14 carrier has a total weight of over 18 pounds, the M-16 comes in at under 14. That's two frag and two smokes, or two quart canteens or a full days rations or...
Four pounds is a LOT if you are humping bush. Even if all you do is carry three more mags you are way ahead and those extra rounds may just bring you home.