Player > Feats > Special Weapon Proficiency
You know how to use a special weapon.
Proficiency in basic melee weapons or small arms.
You gain proficiency in a single special weapon of your choice. If the special weapon you choose is a melee special weapon, you must have proficiency in basic melee weapons. If the special weapon you choose is a ranged weapon, you must have proficiency in small arms. If a set of special weapons are all different models of the same weapon, proficiency in one such weapon grants proficiency with all weapons in the set.
You can take this feat multiple times. Each time you do, select a different special weapon.