AS ISSUED
The M14 isn't controlable in full auto fire AS ISSUED. Not for most people, anyway. I have seen reports of some fairly simple mods to the gun that reduced the fire rate down to around 450-500rpm and that was reported to me much more controlable.
It is conceivable that the military might have also discovered this, given enough time, but political considerations (in and out of uniform) ruled the M16 to replace the M14 too soon.
My first experience firing the M14 on full auto came in Small Arms Repair school. My class was the last class to be trained on the M14. Firing was done at the indoor performance test range. A student was issued a weapon with a number of defects, which they had to find, and repair. Filling out the 2407 form was half the grade! The repaired weapon was cursorily inspected by an instructor, then the student fired it through a small port in the wall into a sand filled room, to function test it.
The firing port was set low, so one had to kneel to use it. We were given a magazine with 15 rnds. 5 rnds semi auto, 10 full auto, to be fired in short bursts.
Being the 4th or 5th one finished, I got to hear the students ahead of me firing "short" burts, of two rounds. Two round bursts are fairly easy to pull off with the M14. Having fired the M16A1 and the M60 in basic, and owning a couple of .308 Win bolt action rifles, this cheeky 18yr old decided to pull off a 3 rnd burst.
What I did not realize, and what the M16 nor the M60 taught me was that in a rifle of that caliber, weight and cyclic rate, is that recoil is cumulative.
I had not allowed for that, and my 3 round burst was 6 rounds, which moved me from a kneeling to a sitting position! My rifle, however didn NOT come out of the firing port. I got a dirty look from the gunny, and the stern reminder "SHORT bursts!", which I did take to heart, firing the remaining four rounds in two round bursts.
In as issued condition, the M14 is not good for full auto fire, beyond short bursts at close range. And as lots of people will tell you, aimed semi auto fire gets more hits per round expended.
Had the military kept the M14 in frontline service and had they introduced some rate reduction to the full auto fire rate, it could well have proven to be a viable successor to the BAR. We'll never know for certain, all we can be certain of is that as issued, it was not a satisfactory full auto rifle.