The MK14 is described in the game as:
A selective fire variant of the M14
You’ll find this in Koani province, at the far north edge of the map in the middle of nowhere. There are only three Unidad and three cartel nearby, having a meeting, and one lone cartel near the weapons case.
It’s easy to take out the guy near the case and just get the gun, but if you want to open up on the others, then once the shooting starts the cartel and Unidad will attack each other and you just mop up.
This rifle has good range and accuracy (though the SR-1 is better), its party piece is the rate of fire.
When you first use it, the gun is full auto (FULL AUTO!) but it has options for three round burst and semi auto as well. I want to control my sniper shots so I leave it on semi auto personally, which makes it an average gun.
You can fit all sniper accessories to the gun and it has a 10 round standard magazine, with a 20 round extended mag available in Media Luna.