Source: JP wiki (notice that weapon's skill page for Stinger still has it under berserk & crit frame).
攻撃する毎に武器毎で発動判定を行
For every Stinger weapon roll will be made.

3本装備していて、全部発動すると+60% になる、といった具合。
With 3 Stingers equipped, if every roll is a success +60% damage will be applied.