Alt-Fire (5e Variant Rule)
From D&D Wiki
Weapons with this property, usually modern or futuristic firearms, have an alternative attack. This property is only available if you are proficient in the weapon and choose the weapon when you gain the weapon alt-fire feature (as either a feat or a class feature).
Classes that have the alt-fire feature:
Weapons with the alt-fire property:
|Argent Plasma Rifle||Very Rare|