Unclear on what object node does

Hello, I am new to UE5 and I have started to make a game and i am using a gun, In this gun i have made an ammo system and have moved on to create pick up ammo. The pick up part of the ammo works but it does not replenish the ammo of the rifle. At they moment i am trying to cast to the rifle blueprint and change the ammo variable to be full but on the cast i am unclear on what to put into the “Object” in the cast