How to Reassemble an AR
In this section, we will provide instructions on how to reassemble an AR, following disassembly, cleaning, and inspection. To reassemble an AR, you will basically reverse the process of disassembly.
Note that this section does not describe how to install certain components (specifically, those that require tools to do so). Refer to the applicable step of the How to Build section for instructions on how to install a particular component. For ease of reference, please find the applicable steps linked in the table below:
Install Lower Receiver Components
Step R1: Reinstall the Buffer System
Slide the buffer spring into the receiver extension. You may need to depress the buffer retainer pin, if the buffer spring gets caught.
Seat the narrow end of the buffer into the buffer spring. Push the buffer past the buffer retainer pin. Note that you may need to depress the buffer retainer pin using your finger or a small flathead screwdriver.
If you are installing a captured buffer spring, the process is similar, but all at once.
Step R2: Reassemble the Bolt Carrier Group
If you have removed the extractor, place the extractor in the correct position on the bolt. Be sure that the extractor spring is installed in the appropriate position. Reinsert the extractor pin into the bolt to secure the extractor to the bolt.
Insert the bolt into the bolt carrier fully, with the extractor facing the right side of the bolt carrier (9 o’clock position when looking at the bolt face) and the ejector on the left side of the bolt carrier (3 o’clock position when looking at the bolt face). The hole in the bolt should be aligned with the cam pin groove cut into the bolt carrier.
Reinsert the cam pin into the bolt carrier and bolt, with the long side of the rectangular head in line with the length of the BCG.
Rotate the cam pin 90 degrees so that the long side of the rectangular head is perpendicular to the length of the BCG.
TIP: If using an index-marked cam pin, orient the mark the same as it was before you removed it. This will minimize wear to the inside of the upper receiver.
Insert the firing pin from the rear of the bolt carrier. The firing pin should insert without resistance. If you meet a blockage, verify the correct alignment of the cam pin. Push the firing pin to the forward-most position in the bolt carrier.
Insert the firing pin retaining pin into the left side of the bolt carrier. Be sure to insert it fully. The retaining pin should not protrude from the bolt carrier. If you encounter any resistance, the firing pin may not be inserted fully or the retaining pin may be bent. In the latter scenario, rotate the pin while applying downward and inward pressure. If that doesn’t work, replace the retaining pin.
Cycle the bolt forward and backward to verify proper installation.
Gas Ring Wear Inspection
With the bolt carrier group cleaned and reassembled, you can inspect the wear of the gas rings. Extend the bolt to the forward position in the carrier. Stand the BCG upright on a solid surface, with the bolt facing down. If the bolt retracts into the bolt carrier under the weight of the BCG, the gas rings are worn out and should be replaced. Go to Repair and Replace for detailed instructions.
Step D3: Reinstall the Bolt Carrier Group and Charging Handle
Insert the charging handle into the upper receiver. The tabs on the charging handle will pass through the applicable cutouts in the upper receiver. Do not push the charging handle forward.
With bolt in the forward-most position in the bolt carrier, insert the bolt carrier into the charging handle and upper receiver. The gas key will seat into the groove in the charging handle stem.
Push the bolt carrier from the rear until the bolt carrier group and charging handle click into the forward-most position.
Step D4: Reassemble the Receivers
With the pivot and takedown pins in the open position, seat the upper receiver onto the lower receiver.
Close the pivot pin.
Close the takedown pin.
NOTE: You should be able to close these pins without any tools. If you have difficulty moving the pin from the open position, add a drop of wet lubricant to the detent channel of the offending pin. Using a non-marring mallet, tap the pin to move it. Cycle the pin a few times to see if the issue resolves itself. If not, head over to Troubleshoot.
At this point, your AR should be clean, lubricated, and back together. As such, you should be ready for a dry-fire function check.