If you wanna modify your existing function, pass a unique index or something into each FirePlasmaRayAlt() call and then adjust the angle based on the index.
Replace CalcWeaponPositionImprecise() with CalcWeaponPosition() and modify plPlasmaRay.pl_OrientationAngle after its call. Or use plPlasmaRay.Rotate_Airplane() to add rotation relative to the current view.