Master of Rifles Achievement

You are required to perform 50 headshot kills with a distance of over 100 meters for each with each of the rifles listed below. Both unlocked and found rifles progress towards this, but the found rifle must be the ‘vanilla’ version; if it is silenced, scoped or powerful it will not count. Best of luck consistently scoring headshots from 100+ meters away with just iron sights.
 

• M.1903 - unlocked by default
• SREM-1 - unlocked by default
• Karabiner 98 - complete kill challenge in Mission 2, ‘Occupied Residence’
• Gewehr 1943 - complete Mission 3, ‘Spy Academy’
• M1A Carbine - complete Mission 5, ‘Festung Guernsey’
• RSC 1918 - complete kill challenge in Mission 7, ‘Secret Weapons’

Unfortunately, the farming method outlined in the 'Mop Up' section of the roadmap doesn't work for rifles as you need to get a headshot from 100+ meters away. A good spot to farm this is at the beginning of Mission 3, ‘Spy Academy’. Building on the solution in The Long Game (15G), mark as many enemies as you can (about 10-12) then create a save. Using the rifle you need to master, proceed to headshot as many enemies as possible then keep reloading the save until you get 50 headshots. Your results will vary, but I was able to get approximately 5-7 headshots per reload, depending on the rifle.
 
The reason this is a good spot is because it’s at the very beginning of the mission, so you can just keep selecting this mission with the rifle you need to master.
 
To check your progress, press then navigate to Service Record > Medals > Weapons and Items and look under the ‘Master of Rifles’ heading to see your medal progress for each rifle.