Killshot is a Companion Perk in Fallout 4 which allows players with maximun Relationship levels with MacCready to earn a 20% increase in headshot accuracy in V.A.T.S.

To get McCready's appreciation, you'll need to make decisions that make him happy. He likes violence and loves watching you steal and break into locked areas, but he's not a big fan of cruel behavior or attacks on the innocent. Keep talking with him as you adventure. Eventually, he'll ask for help with a couple of things and your relationship will max out.