We Have An AR At Home, Part 5: Assembling the Upper Receiver

Now that the lower receiver has been assembled, it’s time to move on to the upper receiver. If it’s any comfort, there’s much less work involved with this portion of the process.

The star of this article is the upper receiver itself. Depending on the style of upper, it may or may not have provision for a forward assist, and the port door may or may not be installed.

There are two distinct general types of AR upper receiver, the traditional A1 or A2 with the so called “carry handle” (it’s not, but that’s a history lesson for another time) and the flat top with Picatinny rail. For those interested in assembling early A1 style ARs, Retro Black Rifle is the best reference I’ve found.

Forward Assist

If the upper receiver is designed for a forward assist, it’s usually necessary to install the parts. I say "usually" because some uppers are available with small parts fully assembled.

1. Secure the upper upside-down in a padded vise.
2. Place the forward assist spring on the forward assist body.
3. Note the notch or relief in the forward assist body. This goes towards the upper receiver.
4. Using a proper size roll pin punch, start the roll pin in the forward assist housing.
5. Insert the forward assist assembly, making sure it is properly oriented, and compress about halfway.
6. The goal is to make sure the roll pin clears the notch.
7. Gently tap the roll pin until the forward assist assembly is retained.
8. Drive the roll pin home until it is flush with the bottom of the forward assist housing.
9. While looking inside the upper receiver, test the forward assist to make sure it moves freely.

Ejection Port Door

This is another sub-assembly that should be done inside a clear plastic bag. The tiny C-Clip can all too easily go flying, never to be seen again. It’s also a good idea to have a spare or two on hand.

In addition to the port door itself, the parts required are the aforementioned C-Clip, port door spring, and port door axis pin.

While the C-Clip can be affixed to the axis pin once installed on the upper, I find it much easier to do this step first. Based on something I saw in a SOTAR video, I designed and printed a fixture to simplify this process. Remember my post on 3D printers?

If installing the C-Clip manually, I again stress the benefit of a clear plastic bag to retain flying parts.

1. Secure the port door axis pin in a vise with soft jaws.
2. Position the open end of the C-Clip against the groove in the port door axis pin.
3. Using a pair of smooth jaw pliers, compress the C-Clip onto the port door axis pin.

Once this step is accomplished, it’s time to tackle the port door assembly.

1. Secure the upper receiver in a vise with soft jaws, ejection port facing up.
2. Insert the port door axis pin from the front of the upper receiver,  with the C-Clip end on your right.
3. Position the port door in the open position and slide the port door axis pin through until it’s just visible in the central gap of the port door hinge.
4. Place the port door spring on the port door axis pin with the long leg to the forward end and against the port door.
5. Using a hemostat or needle nose pliers, grip the short leg of the spring and rotate it around the port door axis pin once clockwise.
6. Be careful, the spring will attempt to escape!
7. Continue to hold the small leg of the spring and slide the port door axis pin through the other half of the port door and through the lug on the upper receiver.
8. The C-Clip should wind up against the front lug.

The core of the upper receiver is now complete. Next we install the  barrel, gas block and tube, muzzle device, and forend. However, that needs to wait until a later post.

In the meantime, good luck and safe shooting.