Master of RiflesG 15

Mastery for the rifles requires getting 50 headshots at 100+ meters away. In the case of the M1 carbine you may want to modify it to perform better at long range, but for the other 5 rifles they’re very capable of 100m headshots from the very start.

The beginning of mission 3 is a good place to get long range kills. Binocular tag a bunch of enemies, drop a save, shoot them, reload the save, then shoot all the tagged enemies again. You can get 50 kills fairly quickly then restart the mission with a different rifle and repeat the process.