From Spud Wiki
This specifically shaped object made of a semi-hard to hard material is used in both tee vortex and inline vortex BBMGs. It is located inside the chamber of the gun and is designed to take BB's located outside of this part and suck them into the barrel of the BB machine gun using what is known as the Venturi effect.
Some people use lathes in order to turn pieces metal (usually aluminum) or plastic for the construction of a vortex block, while others prefer using either Bondo Auto Body Filler or epoxy molded in a PVC plug. Designs for building a Bondo vortex can be found on Burnt Latke's BB machine gun construction page.