MacCready is an optional Companion and can be obtained at Goodneighbor inside The Third Rail, by hiring and paying him 200-250 caps. Helping children/parents, and stealing will help increase your relationship status with MacCready.
Once you gain enough affinity with MacCready, you will obtain his side quest Long Road Ahead.
After completing Long Road Ahead, and reaching the highest level of affinity with MacCready , you will gain the Killshot perk, which gives headshot accuracy in V.A.T.S. increased by 20%.
