The 600 yard range was not the driving requirement. It was the ability to perforate armored personnel carriers. The .30-06 M2 AP could just barely perforate the armor of a BTR-40 and tear up the tires. And that was what we were expecting to come rolling across the German country-side. The British 7mm was incapable of doing that.Great research, great write-up. Thank you Lysander. I have thought about this question quite a lot. I will share my thoughts in the interest of discussion.
First, I think that the Cold War and the Vietnam War played large roles. There is a saying that militaries always prepare to fight the last war, and that is somewhat true in the case of the M14. The biggest threat facing us after WWII was unquestionably the Soviet Union. The worst case scenario is that we would be fighting again in Europe, and quite possibly in the Soviet Union itself. This had a major influence on the new rifle designs. The US insisted on a full power cartridge with this conflict in mind. The 600 meter range of such a cartridge could be used in much of Europe and the Soviet Union. (Whether you could train soldiers to utilize it is a different question, but the potential was there.) A war with the Soviets was also expected to be highly mechanized, and a full powered cartridge is effective against unarmored vehicles. You can see the influence in the M14 rifle too, with its 22 inch barrel to achieve peak velocity and retention of the M1's fine sights.
The war we got instead was Vietnam. There is no doubt that the M14 acquitted itself well there and won many devotees. However, that war was made for short rifles chambering intermediate cartridges. The terrain there usually forced engagements to be 300 meters or less. The Army's own 3 way trial between the AK-47, M16, and M14 showed that both of the others could put more fire on target than the M14 within that range.
Second was the long running program to try to increase hit rate and probability by increasing the number of projectiles a soldier could put in their air. Ultimately things coalesced around the Small Caliber High Velocity concept. Proponents hoped that by flattening the trajectory and minimizing the recoil, soldiers could use rapid or automatic fire to increase hit rate. All of this became embodied in the M16. The theory failed in the field, but we had to try it to find that out.
Third was Robert McNamara and his distaste for all things of government bureaucracy and inefficiency, as he defined them. The M16 was the poster child for his vision of government reform. It was a futuristic weapon at the time. It would be produced by private corporations and undercut bureaucracies he hated. The M14's production issues were an ideal "problem" for him to "solve" with his new vision of government.
I think these are the three main things that doomed the M14. They would likely have doomed any 7.62mm NATO rifle we adopted. As Lysander pointed out, there were problems with the technical data package for the FAL, and the AR-10 was not sufficiently developed, so both likely would have had their own particular teething problems. The push for a new weapon that would increase fire rate was strong, and we were looking to technology to solve the quagmire in Vietnam. So I think the adoption of the M16 was inevitable.
I do not think that replacing the M14 was necessary, and here is where I will anger the Cult Of Stoner. The M16 has served us very well and I would not dispute that. However, the 7.62mm NATO, the FAL, and the G3 / CETME went on to serve in Europe well into the 1980's and 90's, and even to this day in many nations. They have prevailed in conflicts around the world, including against forces armed with Kalashnikovs. I see no reason why the M14 could not have done the same. Europe's replacement of these systems was largely driven by a desire to get back to a common NATO standard cartridge with the US. Forces that continue to use the 7.62mm NATO weapons and cartridge do so because replacing them is not worth the cost, which means they are battlefield effective as far as those nations are concerned. When it comes to ammunition loadout, most of the weight is in the projectile. Therefore, there is not a large difference in weight between 7.62mm NATO and 7.62x39, especially when you factor in the heavy AK magazines (though M14 magazines aren't light).
It is also interesting to note that detractors of the M14 make much of its production problems, while glossing over the M16's problems when it first entered service. At the risk of oversimplifying, both had the same root cause: the inability to produce parts within required tolerances in sufficient quantities with sufficient speed. The problems were solved for the M16. They could have been solved for the M14. TRW got it right, and other manufacturers could have too.
All that said, the M14 has its flaws, and here is where I will anger the Cult Of M14. In my opinion, the biggest design flaw of the rifle as adopted is the traditional stock. I have shot mine extensively, both for fun and in competition, with both the original and the E2/A1 stock. The E2/A1 stock is superior for everything except bringing the rifle to shoulder from rest. The advantage of modern pistol grip stocks was demonstrated in WWII and well known when the Cold War battle rifles were developed. In my opinion, the E2/A1 stock should have been the standard (minus the extra hardware made for LMG use). The bolt roller is a subject much controversy. Eliminating it should have been seriously studied. The attachment of the op rod guide to the barrel should be more solid to eliminate play. Had the M14 remained in service, it would have gone through the same decades of iterative development that the M16 has enjoyed. There is no doubt that it would be a much different and much improved system today.
Would our troops have been better off if we had kept the M14? I don't know. While I don't think that replacing the M14 was a military necessity, the M16 did take a few steps forward in technology. There are too many variables in this question for me to go into in this already long post. In spite of detractors' opinions, I don't think the US military would have been diminished if we had kept the M14.
By the time the M14 was fielded the Soviets had upgraded to the BTR-60, which made the AP question moot, as those were proof against anything smaller than a Caliber .50 AP.
Another thing many people try to do is draw comparisons between the AK-47 and the M14/M16. And that is a mistake unless you start to delve into doctrine. The Soviets (now Russians) never tried to a one-cartridge-fits-all approach. Their doctrine divides the battlefield into two zones, the company zone and the battalion and higher zone. The company level weapons are for the assault and close-in defense, the battalion zone includes the approach and suppression weapons. Because of that neat division, the smaller 7.62mm X 39 and 5.45 cartridges are for company weapons and the 7.62mm X 54R weapons are in the battalion inventory.
And one last thing about the "faults" of the M14:
1) The stock. One of the most common types of shooting in combat is the the unaimed "snap-shot", where a target unexpectedly presents itself and you bring the rifle to your shoulder and shoot without taking deliberate aim. Exactly like trap or skeet shooting. For this type shooting the traditional shock is superior to the pistol grip. In the case in deliberate aiming, such as target shooting, test show it is a draw. People have preferences and tend to do better when using their preferred style stock, but if teaching someone from scratch the average scores are the same. The only real advantage the pistol grip style stock has an advantage is controlling fire in full automatic.
2) The bolt roller. The M1 did not have a bolt roller. The first T20s did not have a bolt roller. they added it because it reduced the stresses in the operating rod during full automatic fire.
3) The end result of the T44E4 shows the fact that it never was intended to be, but rather cobbled together as required. Other examples: the front sight and bayonet lug are mounted on the flash hider , which is held on the barrel by a nut inside a broached hole. That makes the L1A1 with a spacer look positively brilliant. The gas piston requires a flat on the tail so it does rotate, so they have to broach a "D" shaped hole in the gas cylinder, and keep it concentric to very tight tolerance. The 90% of the full automatic trip mechanism is on the outside. Splines, we love them so much we made two parts have them. It was actually rather late in development that they stopped bolting the pivot point of the bolt stop to the receiver.