Think, MASSIVE FIREPOWER!
The skull makes every weapon function essentially as the Scarab Gun functions. Every projectile the weapon fires is replaced with a Scarab beam. For burst-fire weapons and automatic weapons, multiple beams will be fired per trigger pull. However, the Shotgun only fires one beam per shot despite that normally multiple projectiles are fired. This does not affect enemy weapons, and the effect is only applied when the player themselves fire. The Energy Sword is the only weapon not affected.