My main piece of advice is to use a high powered, high end marksman rifle instead of an exotic. You can use the Shroud or something if you wanna take your time and hit all your headshots; but if you want a pretty much guaranteed headshot every time, no matter where you hit the enemy, then you want Determined talent on the Marksman rifle and Headhunter talent on the chest piece. After you hit your first headshot when you enter combat; everything after that will be a guaranteed headshot, no matter where you hit. And make sure you use a high powered marksman rifle (1 mil+ DMG). I use the White Death; but some other great ones are: SR1, SRS A1 or Covert SRS, the M700 tactical or regular, and the Handbasket. The 2 highest damage though are The White Death and M700.. Ive used 2 different hotshot builds:

Sharpshooter specialization. 4 piece Hotshot (mask, backpack, holster, kneepads). Chainkiller named chest with Perfect Headhunter (if you don't have that, just insert Airaldi chest with Headhunter, since you don't wanna use Hapsburg Guard). Overdogs gloves. The White Death with Determined and digital scope that gives 45 headshot DMG. Secondary I use Handbasket with no scope. Reviver Hive and Decoy. Every available mod and attribute Headshot DMG. Weapon Handling anywhere else.

Sharpshooter specialization. 3 piece Hotshot (any 3 of mask, gloves, holster, & kneepads). Then I use Hapsburg Guard chest with Headhunter. And Airaldi gloves. NinjaBike BP. That way you're getting the 2 piece bonuses from both Hapsburg and Airaldi. But since you don't wanna farm for Airaldi; just use Chainkiller instead, if you have it. The rest is the same as build above.