WEAPON PROFICIENCY (FIREARMS) is a former Physical Skill which improves accuracy and damage with firearms. It was taught to AKEMI HOMURA by TOMOE MAMI on Loop 1. It was deprecated in a patch, splitting proficiency along different sizes and types of firearm and folding in the firing/reload rate benefits of STANCE (GUNFIGHTING).
WEAPON PROFICIENCY (FIREARMS) appears to remain on Mami's character sheet as displayed by IDENTIFY, but presumably remains unavailable for Homura.
Passive: +1 to hit with firearms. +20% damage from firearms.