Treyarch’s David Vonderhaar has announced that they will increase the XP earned per kill in Black Ops 3’s Search and Destroy mode from 250 to 500. However, he didn’t reveal when the update will start rolling out.
@ernestandrei It's coming. Going to 500 from 250. XP, not score.
— David Vonderhaar (@DavidVonderhaar) November 10, 2015
As for raising your XP rewards, changing weapons from time to time is one the most effective ways. You receive XP rewards for every tier you finish after getting a certain amount of kills and assists.
Try using some of your classes for weapon types you don’t frequently utilize, and you can earn up to 500 XP for getting 100 kills with one.
Back to Vonderhaar’s confirmation, we’ll update you as soon as the story develops.