AR and Handgun kills shouldn't be hard to come by, and if you farmed other perks, you'll have them by the end of Chapter 2. The Marksman rifle kills won't be as straight forward. However there's a good spot to farm these early in the game.
Early in Chapter 3 you will exit the car and have to clear the area. In front of you and to the left are 2 large buildings connected by what appears to be a zipline. On the roof of the far building (which can be accessed from the back of the building) is a Marksman Rifle leaning on the wall next to a window you can use to kill the commander.
Grab this rifle and clear the area. When you move forward with the car a checkpoint will be reached and you will need to enter the bunker on the left to proceed. There are 4-6 enemies in here, and you can kill them from outside the door and then reload the checkpoint to farm these kills.