It may very well have been an M-14 rifle that Sgt Davis was using, but if so, it was most likely the match-grade sharpshooter version designated M-21.
M21 is a Vietnam era sniper system. What Davis has is a regular grade M14. A quantity of them have been supplied to the Stryker units for designated marksmen. The only modification is a rails system for mounting optics and other ancillary devices.
They were also given special training using a contract instructor who is an expert combat shot (and retired officer). Sgt Davis attended this training last year.
d.o.l.
Criminal Number 18F