Barreled AR Upper Receiver
Headspace go and no-go gauges that match the chambering of your rifle.
Most barrels are shipped with the barrel extension in place, and in theory everything is machined so the headspace will be correct when the bolt is closed. But, it’s a very good idea to double-check to reduce the risk of experiencing a spontaneous separation of parts the first time you drop the hammer on a live round.
Run through a few dry cycles to make sure the bolt cycles. Close the bolt and shut the dust cover. Now, pull back on the charging handle, and the dust cover should open. Make sure the forward assist pushes against the bolt when you press the plunger and releases cleanly. Insert an empty magazine, and check that the bolt locks back. Also make sure the bolt release functions properly. Finally, clean the bore and chamber and install some sights and/or an optic.
Your completed AR upper is now ready to be joined to a lower receiver by attaching the pair by the front-pivot pin and the rear-locking pin. What’s that, you down own an AR lower? (What a rare breed you are, indeed.) The good news is building one is easier than the upper. However, (spoiler alert) that sounds like the topic for another build, doesn't it? Stay tuned.