In order to get the six Rifle Mastery Medals, you need to get 50 Headshot from 100 meters away with every rifle in the game.
To help you guys, with this guide, I'll leave here a link to my Twitter, with a video and an image of one of the best places in the game to get headshots from 100 meters away.
Mission 2: Occupied residence.
https://twitter.com/EikeMendes1991/status/153323661993063219...I hope this guide can help you.