Weapon proficiency

 show how effective your character can be with each weapon type. You gain proficiency for a given weapon type every time an enemy is killed while said weapon is your active weapon. You can check your current proficiency levels on the character status page.

Higher proficiencies should lower level requirements to equip guns of that category, but a gun's requirement is lowered only after it has been equipped. This is a feature that was removed from the game prior to release.

Bonuses
Each proficiency level gives you three bonuses for every weapon in that category, which affect different weapon characteristics:
 * Unless otherwise noted, guns receive Damage, Reload Speed, and Accuracy bonuses.
 * Sniper rifles replace Bonuses #2 and #3 with Rate of Fire and Stability.
 * Eridian weapons replace Bonus #2 with Cooldown Rate, which determines how quickly its magazine recharges.
 * Shotguns replace Bonus #3 with Rate of Fire.