According to Gefeios, you should upgrade your weapon after your skills have been increased. That is why you obtained the Essence of the Black Storm, Special Shulack Refiner and the Blessed Aurium Ingot and brought them to the Jotun Studio together with Siel's Gleaming Magic Weapon. The components were made into the Crystal of Siel's Essence. Upon bringing Siel's Essence to Gefeios, he created Siel's Dominant Magic Pistol.