Zeus Arms "Hollow Apollo" AR-15 Lower Receiver. The Hollow Apollo series lower features lightweight skeletonized cuts to give assist in weight reduction and allow quick access to the trigger housing to clear out any malfunctions due to faulty ammo. It also shows no visible markings on the sides of the receiver maximizing the amount of space for customization. All of our lowers feature a more aggressive magwell flare and a upper retention threaded hole. a 7075-T6 aluminum a Precision Machined Magazine Well a Accepts all mil-spec components a Upper Retention Threads Built in so you can always get a snug fitment