I first fired an M14 in 1957. I immediately concluded that it was a nice semi-auto rifle, and absolutely flicking worthless in full auto. Later, I was a member of the old Army Ordnance Assn. (now, I think it's called the American Defense Preparedness Assn.). At one of their meetings in Washington, they arranged for busses to take groups to Aberdeen for a weapons demo. At that time (1959), there was a lot of controversy about the M14. The Army told us they would demonstrate that the M14 could be controlled in auto fire. They trotted out a couple of master sergeants with hash marks to kingdom come. These guys were big and wide. Each one looked like the Redskins line - not a lineman, THE LINE. They picked up two M14s (which looked like Mattel toys in those hands), fired full auto, and guess what? No movement. What did you expect with what looked like 600 pounds of beef holding on? So, if you are built like that, the M14 is controllable. Otherwise...
Actually, that is the major reason the army went to the M16, to get controllable full auto fire. Savings in weight, material, and ammo cost were important but secondary to the need to counter the AK-47, both in combat and in propaganda and PR.
Jim