As of 10/08/2018 this achievement does not track properly - however it is still possible to obtain.
The only assault rifles that appear to track kills towards this achievement are the M16A4 and the M416. The distance (100 meters away) is not required, feel free to kill enemies from any range; what you MUST do - to progress this achievement - is make sure you are aiming down the IRON SIGHT. I have tried aiming with a red dot sight on numerous occasions and not once has it tracked the kill. 6x scopes appear to work on occasion but they are not consistent. At this time the only sure fire way to progress this achievement is using the weapons iron sights.
I have not been able to test every sniper rifle but can confirm that the VSS, Winchester, Mini 14, and M24 do not count towards this achievement, even from a distance greater than 100 meters.
One final note, kills against AFK players in the water do not count towards this achievement.
(VictimOfDesire noted that the achievement progressed using the 4x scope. In my experience all scopes have been inconsistent and as mentioned above the red dot has never progressed. However it may be worth a try, scopes may work for you. It seems to be a case by case basis.)