Very often down through the years when ever the Army has wanted to “Reach out and touch someone” they have called up the M-14 as the weapon that is fundamentally up to the task.
Unfortunately the M14 doesn't lend itself to extreme accuracy. The rifle, by its very design, is kind of a poor platform for long range work. Scopes are something of an after thought, the gas system needs to be modified, a heavier match grade barrel added, the stock replaced and bedded. Even then, with heavy use, the rifle begins to rapidly loose accuracy and needs to be sent back to the armory for some TLC.
The M14 was pressed into the role of designated marksman because it was already in the armories. It's by no means the ideal solution to the needs of our military.