Federal 5.56x45mm 50gr Semi-Jacketed Frangible loading, XM556NT1.
Your are correct, the headstamp is " F C 15 " i.e. Federal Cartridge Company and produced at Lake City Army Ammunition Plant (according to the info I have) in 2015.
Typical contract (overrun) box:
Typical contract ammo can.
The " N " marked primer indicates a NON-Toxic primer.
Ron Fuchs lists a number of different versions of the semi-jacketed frangible loadings by Federal in the Federal Cartridge Corp. section of his book on 5.56-.223 ammunition