As of 10/08/2018 this achievement does not track properly - however it is still possible to obtain.
The only assault rifle's 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.
Note: As of 06/21/19 it sounds like this achievement still tracks incorrectly. I want to reemphasize that distance does NOT track correctly and iron sights appear to still be your best bet - thanks andresfgp13 for reconfirming that. I wanted to clarify that I no longer play PUBG and am unaware of recent updates/changes but wanted to leave this solution up because there are helpful comments below.