This is unlocked by fully leveling a weapon skill to 20. For example, use Assault Rifles until your skill in them is 20. Each weapon has an XP bar that is filled by doing damage with the weapon. The XP earned with the weapon adds to the weapon skill. Once that XP bar is filled, you need to swap to a new weapon to continuing leveling the skill. The little white bar under the weapon and ammo part of your HUD displays the current weapon skill level progress. To check the progress of all weapons, open your Start menu and use or to tab over to your Stats menu. As you scroll down, you will see the progress bars and current levels of all weapon types.