If you're building a rifle from scratch or installing a barrel to your upper receiver it is important to check the headspace of your rifle. This clip demonstrates how to check headspace and how to install your ejector. PDF instructions also available.
WARNING: Never attempt to disassemble or reassemble a firearm unless you are absolutely certain that it is empty and unloaded. Visually inspect the chamber, the magazine and firing mechanism to be absolutely certain that no ammunition remains in the firearm. Disassembly and reassembly should follow the manufacturer’s instructions.