Laser Sight increases damage with critical hits for 15%. It works for any weapon since all can make critical hits, even Elementals (despite it shooting out clumps of elemental stuff). You can use it best to additionally power weapons that already have increased chance for doing criticals. It will increase their damage. For example, Tequilas have low damage, but increased chance for doing criticals, so with Laser Sight their damage is better.
Tip: Use a Gunslinger for this attachment since their critical chances are high.