Im guessing you are just filling up the Marksman Tree, I believe by attaining Apprentice Sharpshooter, Apprentice Gunfighter, and Apprentice Carbineer, that the ratio of Weapons XP and Combat XP you should get the Ranged Support Tree as soon as you complete the first 3 trees of Marksman.
3 words to get Ranged Support 4...
KILL KILL KILL