The Tools You'll Need
The upper receiver goes together quicker and easier than the lower. Get all your tools organized, including your safety glasses. Here are all the tools you'll need: needle-nose pliers, a selection of hammers, drift punches, roll pin starter punches, a center punch, if staking the carrier key, lithium grease and light oil, a torque driver like this Wheeler Fat Wrench for the carrier key screws, an action block to hold the receiver while installing the barrel. You'll also need a 1/2 inch torque wrench with an adjustment range from 30 to 80 foot-pounds, anti-seize compound, and snap ring pliers.
Specialty tools include a gas tube alignment gauge, front sight tool, and a rear sight installation tool like this one. Some dummy rounds are handy for function-testing. Headspace gauges, a firing pin protrusion gauge, a magnet, and cleaning solvent.
Lastly, you'll need a handguard tool and a quality screwdriver set like this one from Wheeler Engineering.
The Ejection Port Cover
Now that you have all the tools laid out and organized, let's get started on the upper receiver. First, let's install the ejection port covers. This is a simple job, but always go slow and be patient, thinking about what you're doing.
This tiny E-clip secures the rod that holds the ejection port cover and spring to the upper receiver. It's hard to hold and easy to lose. The best way to hold the clip for installation is with needle nose pliers. Position the E-clip over the groove in the retaining pin and tap it into place with a hammer. It seems easy if you do it right the first time.
Start by oiling the ejection port cover pin. Now, with the upper on its side, start the smooth end of the pin through the right receiver boss and then to the right side of the cover.
Now lay the spring into position with the long end on the right side pointed up. Holding the left side of the spring, rotate the right side of the spring down and insert the pin all the way through the spring. Now test the cover for proper function.
The Forward Assist
The forward assist is next. It's important to use a good receiver holding fixture to prevent damage from the vise. The forward assist assembly is made of only three parts: the forward assist, a return spring, and a roll pin. Be sure to oil the roll pin and grease the forward assist and spring.
Start by securing the receiver in a holding fixture and clamp the fixture in a vise. Slip the return spring onto the forward assist and insert the assembly carefully into the hole. Push in and rotate as necessary so you can see through the hole for the securing pin. Use a 3/32 punch, insert it from the bottom as a slave pin. Start the roll pin into position with a roll pin starter punch, then use a standard roll pin punch to drive it the rest of the way home. This will drive out the slave pin. The forward assist assembly should move freely back and forth.
The Rear Sight
Before you begin, it's a smart play to lay out all of the parts so they're easily accessible.
Start by screwing the rear sight base index screw into the rear sight elevation index wheel with the 1/16 inch hex wrench. This screw should be flush with the outside of the index wheel.
Now combine the index wheel with the elevation knob. The index screw can rest in any hole in the elevation knob.
A detent ball places tension on the elevation knob, holding the elevation adjustment in position. Put a dollop of grease in the detent hole and install the spring and detent ball. The grease will hold the ball in place.
Slide the elevation knob assembly into the elevation knob slot in the receiver. The detent ball should seat in any of the holes on the bottom of the elevation knob, holding it in place.
Now put a bit of grease in the rear sight base tensioner hole then slide the tensioner spring into the hole and put a bit more grease on top of the spring.
Put the detent ball on top of the spring.
Insert the sight base into the sight base hole in the receiver and screw the sight base onto the elevation knob. Don't dislodge the detent ball as you do this.
Screw the rear sight base all the way in.
Now place the elevation spring in the hollow cavity of the rear sight base.
To install the sight-based roll pin, you need a rear sight installation tool. It allows you to compress the elevation spring while you drive in the roll pin. Take the body of the sight installation tool and slide it into the receiver from the rear so the hole in the tool body is aligned with the cavity in the threaded portion of the rear sight base.
Insert the fork spring depressor into the body of the tool and compress the rear sight base elevation spring.
Align the small hole of the shaft with the small hole in the center of the body of the sight installation tool. Slide the tool lock into the tool body and through the shaft of the spring depressor. This will hold the tool in place and keep the rear sight elevation spring properly compressed.
Use a roll pin starter punch to start the roll pin in the pin hole. And finish seating the roll pin with a standard roll pin punch.
Now you can remove the installation tool. Lubricate the rear sight flat spring and place it in the recess on top of the rear sight base. The hump, or curve, of the spring should be up.
Next, place the aperture on the spring. The large aperture hole should be up and you should be able to read the number 0 to 2 on the back of the sight. Thread the windage knob screw into the aperture from the left side of the base.
We've chamfered the leading edge of the screw to make threading it easier. Turn the screw until it's fully seated and adjust the aperture until it's located in the center of the sight base.
Now prep the windage knob. Put a bit of grease in the detent hole on the rear face of the knob. Slide the detent spring into the hole and follow up with a bit more grease on the exposed end of the spring. Place the detent ball on the end of the spring.
Now take the knob and slide it on to the right side end of the windage knob screw. Be careful and don't drop the detent ball.
Align the pinhole in the windage knob with the pinhole in the windage knob screw and use a small punch to hold them together.
Use the roll pin starter punch to begin seating the rear sight roll pin then follow up with the roll pin punch to fully seat the pin. The pin should be seated to an equal depth from each side of the knob.
Once the sight has been installed and checked, you should adjust to proper zero. Start by rotating the elevation knob until 8/3 is visible and centered on the left side of the knob.
Next, use a 1/16 inch Allen wrench to loosen the rear sight index screw and disengage the elevation knob from the indexing knob.
Now rotate the elevation knob without moving the indexing knob so the rear sight is at its lowest setting. I like to move the sight up two clicks from the bottom so I have some cushion in my adjustments. This is referred to as a Santos method for Battle Sight Zero. Once everything is set, tighten the index screw to lock the two pieces together.
Well, it should feel good having that job behind you. Hope you don't have any parts left over.
Now is the best time to check headspace. If it isn't correct, you need to fix it before proceeding. Start with a stripped bolt. It needs to have both the extractor and ejector removed to prevent any false readings when using the headspace gauges.
A Go and a No Go gauge are needed for this process. The Go gauge is sized to the minimum allowable head space while the No Go is over the max.
Because fouling brass shavings or other debris can affect the measurements, you must first clean the chamber thoroughly.
Insert the Go gauge into the chamber.
Now slide the bolt into the barrel sleeve. You should be able to turn the bolt and engage the locking lugs completely. This lets us know that the chamber has at least a minimum amount of headspace.
Now repeat the process using the No Go gauge. You should not be able to turn the locking lugs completely in the battery. If you can, then you have excessive headspace and should not fire the gun. A different bolt may correct the problem. Otherwise, you will need to replace the barrel. In either case, the gun must pass the headspace test.
Most bolts come fully assembled, but if yours isn't, this is how you assemble one from parts. I like to start by lubricating all the pins with some light gun oil. This will make them easier to install. Also be sure to add a bit of grease to each of the springs.
Place the extractor spring into the spring recess in the bolt with the large end inside the recess.
Now place the extractor on the bolt and clamp it into place using a padded machinist vise.
Place the roll pin in the starter punch and begin driving the pin through the bolt, switching to a roll pin punch to drive it the rest of the way.
Insert the ejector spring into the recess on the bolt face.
Align the clearance cut on the ejector with the ejector pinhole and insert the ejector into the bolt.
Press the ejector in with a flat face punch. Once the clearance cut is in line with the ejector pinhole, insert a small punch to capture the ejector.
Now take the roll pin starter punch and seat the roll pin. Use a roll pin punch to finish seating the ejector roll pin. As you drive in the roll pin, you should drive out the small punch you use to capture the ejector.
Now slip the ring into the groove.
Repeat this process with the remaining two gas rings then adjust their positions so the open ends are not aligned, thus not compromising the gas seal.
The Bolt Carrier Key
The bolt carrier often comes completely assembled, but here's how you assemble one from component parts. The carrier and carrier key must be securely joined. Because loctite is not strong enough to hold the screws in place, we'll need to stake them. First, install the screws.
Then, using a torque driver like a Wheeler Fat Wrench, torque each screw to 35-inch pounds.
To stake the screws in place, use a center or staking punch and a large ball-peen hammer. Stake each screw twice.
The Bolt Carrier Assembly
Now, let's check firing pin protrusion and build a bolt carrier assembly. Pretty simple if you've got the right tools. Measuring firing pin protrusion is pretty straightforward with the AR. Start by inserting the firing pin in the bolt.
Now zero the firing pin protrusion gage by loosening the set screw and pressing the gauge against the flat surface then retighten the set screw.
Measuring the gauge with a caliper should give us a reading of nine hundred thousandths.
With the firing pin fully seated in the bolt, loosen the set screw and press the gauge against the bolt face. Now tighten the screw and measure again.
Subtracting our first measurement from our new reading gives us 30 thousandths of firing pin protrusion: well within the acceptable range of 28 to 36.
Now insert the bolt into the carrier with the extractor to the right side.
Align the cam pin hole in the bolt with the cam pin slot in the bolt carrier install the cam pin into the carrier with the head lengthwise along the carrier.
With the cam pin fully seated rotate the head 90 degrees. The cam pin should move freely in the cam pin slot.
Insert the firing pin into the rear of the bolt and push it as far forward as possible.
Insert the firing pin retainer into the hole on the left side of the carrier. Make sure the retainer is fully seated in the recess.
Pull on the rear of the firing pin to make sure it's secure. Engage the tabs on the charging handle with the cuts in the carrier raceway, sliding the charging handle in slightly.
Now install the carrier, pushing it all the way forward.
The Delta Ring
Most barrels come partially assembled. Let's take a look at the parts that should be included.
The barrel assembly is made up of the barrel, the barrel sleeve, the barrel nut, the gas block and front sight base, and taper pins. Because installation of these parts requires extreme precision or pressure, most manufacturers perform this work for you. Let's start with this partially assembled barrel.
Before we can attach it to the upper receiver, we need to install the Delta ring, the weld spring, and the retaining ring. Slide the Delta ring onto the barrel from the rear over the barrel nut, followed by the weld spring.
The retaining ring is last. Slide it onto the barrel nut and, using a pair of snap ring pliers, open the ring, engage it in the groove on the barrel nut, and release the pliers. That's all there is to it.
To install a barrel properly, you need a good holding fixture, a barrel wrench, a torque wrench, and some muscle.
Coat the threads of the barrel nut and receiver liberally with anti-seize compound. This will prevent galling when we tighten everything in place.
The top of the barrel sleeve has an indexing stud which mates with a groove in the receiver, assuring the gas port is indexed to top dead center. Slide the barrel into the receiver, and screw on the barrel nut just until it's hand tight.
A barrel nut wrench is used to tighten the barrel nut in the final position while a torque wrench is used to control the tightness. Proper torque is 30 to 80 pounds, according to the Marine Armorer's Manual.
Starting at 30 pounds, tighten the barrel nut until the wrench reaches torque. Then, using a breaker bar, loosen the nut and back it off slightly. Repeat the tightening and backing off process two additional times. This allows the threads on the receiver and barrel nut to properly mate.
Now we can tighten it to spec for its final positioning. Using a small punch, align the openings in the Delta ring and weld spring.
Now, using a gas tube alignment gauge, check to make sure the barrel nut is aligned properly to allow the gas tube to enter the receiver.
The gas tube should not touch the barrel nut. If the barrel nut is not properly aligned, increase the torque a little at a time until it is, but don't apply more than 80 pounds of torque.
Now we can install the gas tube. Insert it five or six inches into the receiver then pull it forward inserting it in the front sight base. Be sure that the gas port is facing down and the roll pin holes line up in the front sight base.
Install the gas tube roll pin in the sight base using a roll pin starter punch and drive it the rest of the way home with a standard roll pin punch.
Now install the carrier assembly and check to make sure that it closes flush with the back of the receiver and the gas tube doesn't bind in the key.
The gas tube should enter the carrier key freely and without any binding.
The Front Sight
Installing a front sight is an easy job if you have the proper installation tool. Start by lubricating the front sight spring and detent with the dab of lithium grease.
Then place the spring and detent into the recess in the front sight base. The spring fits inside the cavity on the bottom of the detent.
Now screw the front sight in until the detent prevents you from going any farther.
This is a frontside adjustment tool. The prongs on the front engage the notches in the sight, forcing the detent down allowing us to adjust the front sight to the desired height.
Simply place the tool over the sight and turn it clockwise until the bottom of the sight is flush with the base, for now.
To install the compensator, all you need is a combo wrench and a vice. This is a standard A2 compensator. With the barrel and a vise, slip the crush washer over the muzzle and screw the compensator on by hand.
Turn it to final position with the center-cut straight up using a combo wrench.
Now we're ready to combine the upper and lower receivers.