Not entirely sure if this is a bug or intentional - but it doesn't feel like its supposed to work like this.
I believe the only thing it has an effect on is the Light Precision power, losing any stacks unless all 3 projectiles hit.
If this IS intentional and each projectile is considered its own attack - there seems to be an inconsistency caused by it, being that the Light Precision and 1v1 powers do not gain stacks for each projectile that hits, instead only 1 stack per shot.
Equip Shellshot Blunderbuss cannon
Pickup Light Precision power
Fire light attacks close to a target - All 3 projectiles should hit and you will gain 1 stack of this powerÂ
Fire light attacks far from a target - 1 projectile should land and gain 1 stack, immediately being lost as the other 2 projectiles timeout