Head into Marcus's Shop. You will find him behind the counter as always. He will give you an assault rifle (it is likely of lower level than the others you've gotten).
Marcus: Jack's monopoly on weapons is running me out of business. But that's not why I want him dead. I want him dead because he's a greedy, murdering sonofabitch who needs to die screaming. Good luck.
Marcus: Here - you might need this.