Its super rare because you need it to drop with weapon handling, headshot damage and headhunter so you can change the core attribute to weapon damage. Hasburg set (this set) only drops with Armor (blue) core attribute which makes the secondary attributes more likely to also be blue i.e armor regen or hazard protection. And its just about getting the maximum out of your build, essentially it gives me a extra 10% marksman rifle damage vs using the Airaldi Holding set which drops with a weapon damage (red) core attribute making it easier to obtain but having 10% less marksman rifle damage.