You can gain more perks in Call of Duty: Ghosts by choosing not to use some slots. Each of the primary weapon, secondary weapon, lethal and tactical slots gets you one extra perk point. Note that not choosing a point streak will not gain you extra perk slots.

Best Gun: http://wp.me/p1SfP0-1EW
All guides: http://wp.me/P1SfP0-1CA