Spikes Tactical M4 Flat Top Black Upper Rifle Receiver - The Spikes Tactical M4 Flat Top upper receiver is machined from 7075 T6 Aluminum and has a tough Type III Hardcoat Anodize finish. The laser engraved T-Marks and M4 feed ramps make this upper true to form. It's ready to accept a standard AR-15 barrel with the pre-installed forward assist and ejection port door cover. Ejection Port Door Spring Ejection Port Door Rod Ejection Port Door Forward Assist T-Marks