Lower parts kit, which includes the trigger, disconnector, magazine, grip, hammer, bolt catch/release, and safety
Let’s look at these parts in detail. We’ll summarize the parts and the important specifications.
Upper Receiver Parts
The AR-15 barrels determine the shooting range, the accuracy, how far you can shoot, and the caliber you are going to use. While choosing a barrel, consider:
Length – This determines the speed of rounds. Short barrels are easy to maneuver and are ideal for use with large muzzles. Longer barrels boast more accuracy and more velocity. In general, a 16-inch barrel will work fine.
Materials – The 4140 Chromoly is the affordable option whereas the 416R Stainless Steel has unrivaled accuracy. The 4150 Chromoly Vanadium (CMV) is the popular choice. Most of these materials meet the minimum requirements.
Manufacturing and rifling methods – Rifling refers to how twisted ridges have been cut into the barrel to allow a bullet to spin for stability and accuracy. There are several rifling options, including cut, button, and broach rifling. Each has its strengths and weaknesses.
Chrome lining – A chrome lining on a barrel increases its life and performance when the gun is fired rapidly. A popular option for this is melonite.
Twist rates – The twist rates for the AR-15 range from 1:7 to 1:12. This ratio denotes the number of times a rifling completes a single rotation per inch. Almost all rounds bought in stores are 55-grain or 62-grain. The ideal twist rate for such is between 1:7 to 1:9 twist.
The gas system is responsible for the rifle cycles and ammunition feeding. The hot gas from each fired shot in your AR-15 forces the bolt carrier group (BCG) back, ejecting the spent cartridge and chambering a new round.
Gas systems are available as direct impingement or piston gas systems. Although they function the same, the direct impingement type pushes dirty gas back into the receiver whereas the piston gas ejects the gas outward.
The gas blocks, on the other hand, are available in four options; the pistol gas block (four inches), the carbine (seven inches), the mid-length (nine inches), and the rifle (12 inches).
For a typical 16-inch rifle, a mid-length system is more appropriate. This choice is reliable and comfortable in terms of recoil. For a pistol between 11 inches and 14.5 inches, a carbine gas length system will work best. For any barrel below 11 inches, a pistol-length will suffice.
Handguard options are available in two: the free-float and the drop-in handguards. A free-float handguard is a better choice if you want to negate the pressure on the barrel since it doesn’t attach to the barrel or the gas block. Military AR-15 rifles mostly use the drop-in handguards since they secure firmly on the gas block.
If you need attachments on your gun instead of a flat-face handguard, then you must choose one of these attachment systems – the M-Lok, Picatinny, and Keymod. These systems allow you to attach other accessories, including flip-up sights, lasers, and flashlights among others.
Since most AR kits feature free-float handguards, M-Lok or Picatinny systems come highly recommended.
Bolt Carrier Groups and Charging Handles
Bolt carrier groups (BCGs) are standardized in the AR-15 world. This group functions by allowing the firing pin to hit the cartridge in the chamber while ejecting the spent shell casings before it chambers a new round. If, for example, you are building a typical 5.56/.223 shooter, then your choices will be between the standard AR-15, M16, or Full-Auto BCGs.
Consequently, the charging handles are also universal. Most of them get the job done. However, you can also choose custom ones with extended grips.
The AR-15 uppers come in various styles, including the A1/A2/A3/A4. These numbers refer to the designs of the upper receiver with A1 being the earliest form and A4 being the contemporary type found on most AR-15s.
Worth noting is that the A2 upper receiver comes with a handle that works only with specific types of scopes and mounted optics. The A4, on the other hand, supports many types of mounts.
For your AR-15 building, you can use the M4 upper. It has a picatinny rail ideal for various quick detach mounts and scopes compatible with the AR-15.
You can also opt for the basic stripped upper receiver. It houses the BCGs and the barrel and is easy to customize. Most of them are universal and ideal for building an AR-15 gun.
Lower Receiver Parts
Stripped Lower Receiver
The stripped lower receiver is the only part of the AR-15 considered a firearm under federal law since it can function with a trigger. You can use this option for your AR-15 building or buy an 80% lower-receiver blank that needs fabrication for it to accept a trigger and the parts kit. If this is your first time building an AR-15, then go with the stripped lower.
These receivers typically have standardized dimensions, and they use the same trigger, parts kit, buffer tube, and pistol grip for the same caliber.
Lower Receiver Parts Kit
The most challenging part in assembling the parts for an AR-15 is putting together the lower parts kit. The components include a grip, safety lever, operating springs, trigger, hammer, bolt catch and release, buffer retainer, magazine catch and release, and takedown pins.
Although installing them is pretty much tedious, the process is straightforward. These parts don’t vary in their design or build. As such, everything should fit in place perfectly.
The Buffer Assembly
The buffer assembly incorporates the buffer tube, buffer, and the spring. The buffer controls the BCGs recoil as well as the buttstock. Buffer tube diameters are specified with words such as "commercial" or “mil-spec."
Mil-spec components are usually forged, but the commercial types are made from billet. This is what differentiates their diameters. The commercial receivers have a buffer tube with an outer diameter of 1.17" whereas the mil-spec receivers use a 1.14" outer diameter housing. While the two are not interchangeable, they still use similar recoil spring and buffer.
Regarding buffers, they are available in different weights. The standard buffer, also called a carbine buffer, weighs exactly 3 ounces. Other common buffers to choose from, including heavy, or "H" (3.8 ounces), H2 buffer (4.6-4.7 ounces), H3 buffer (5.0-5.4 ounces), pistol buffer (5-8.5 ounces), and rifle buffer (5.0 ounces).
The recoil springs in all AR-15s are universal.
Putting It All Together
Before you start building your AR-15, make sure that you have a schematic to utilize as a guide in making the parts assembly. Also, arrange all the tools you need on the workbench and all the parts on a mat. Then group all the parts according to their category.
For the sake of this tutorial, we will assume that you are a novice who just bought the lower parts already assembled.
The following steps are for assembling the AR-15 upper receiver parts.
Building Your AR-15
Step 1: Installing the Forward Assist
Install the forward assist assembly turning the upper receiver upside down to align the Picatinny rail against the work surface.
Slide the spring on the forward assist and insert it to the receiver with the pawl oriented towards the middle.
Slide the forward assist assembly into the upper receiver.
While holding the assembly firmly, drive the pin through until it is barely sticking up. Use a small punch for this task and to ensure that it is flush with the receiver.
Step 2: Installing the Ejection Port Cover Assembly
Place the pin snap ring in the ejection port cover pin’s groove on the end and gently hammer it the rest of the way
Lay the receiver on its side such that the ejection port is facing up and the barrel threads pointing right. Start the ejection cover port with the end that has no snap ring going first.
Place the ejection port cover on the upper receiver with the holes coinciding with the ones on the receiver.
Slide the ejection port cover pin through the hole closest to the barrel in the upper receiver, stopping right before it passes the mid-opening of the ejection port cover.
Hold the ejection port cover spring with your left hand and take the long part of the spring sticking out and wind it half a revolution.
While holding it under tension, take the long part of the spring and place it on the right side of the ejection port cover, and while still holding it, slide the ejection port cover pin through till the pin snap ring stops the progress.
Snap shut the ejection port cover then reach inside the upper receiver and push the port cover, ensuring it opens with a snap.
Step 3: Assemble the Bolt Carrier Group
Push the bolt through to the end of the bolt carrier, ensuring that the holes in both components are aligned.
Then insert the bolt cam pin through the hole in the bolt carrier. Once inserted, rotate the cam pin 90 degrees (one-quarter turn) to align the large holes in the bolt cam pin with both ends of the bolt carrier and to allow the firing pin to slide through.
Insert the firing ring through the carrier until it stops then insert the retaining pin through the bolt carrier and into the hole on the other side of the carrier before it emerges. Ensure that the round end of the pin snugly fits the recess on the side of the Carrier.
Test the bolt to ensure it moves in and out of the carrier freely.
Step 4: Assemble the Charging Handle
Insert the charging handle latch spring into the charging handle, then start the roll pin atop the charging handle. You can use a roll pin punch for this process.
While compressing the handle latch, align the holes and drive a roll pin through, and you are done with the charging handle.
Step 5: Install the Charging Handle and Bolt Carrier Into the Upper Receiver
Place the upper receiver upside down with the picatinny rail against the work surface, and insert the charging handle and slide it in. Make sure the grooves and tabs on both components are aligned.
Pull out the bolt as far as it will go, insert the bolt carrier assembly into the charging handle, ensuring that the bolt carrier key rides the groove on the underside of the charging handle. Stop when the carrier and charging handle are parallel to each other.
Then push the bolt carrier group and the charging handle into the upper receiver until they click firmly. The ejection port parts should open.
Step 6: Install the Upper Receiver Into the Action Block
Detach the charging handle and the BCG from the upper receiver and slide the action block insert into the back and close the ejection port cover.
Place the upper receiver into the action block and close it tight using a vise.
Step 7: Install the Barrel
Push the barrel through the upper receiver where it is threaded and slide it until the pin hits the back of the slot.
Get rid of the coating from the barrel threads using a steel brush then remove the nuts from the free float tube handguard and open and separate the inner barrel retainer nut from the outer receiver nut.
Coat the threads with white lithium grease to prevent them from galling.
Then thread the outer receiver nut on the barrel until it is in contact with the ejection port cover pin.
Unscrew the outer receiver nut and stop when the gas holes on both the nut and upper receiver are aligned.
Then coat the inner barrel retainer nut inner threads, and while holding the outer receiver nut to ensure the gas holes are open, thread the nut and tighten it.
Attach the takedown wrench to the inner barrel retainer nut, ensuring that at least two of its pegs are snugly injected into the gas tube grooves on the inner barrel retainer nut and tighten them.
Then install the torque wrench into the port in the takedown wrench and check to see if the gas tube can slide through the gas holes easily.
Step 9: Install the AR-15 Gas Tube and Gas Block
Insert the gas tube through the gas holes, ensuring that the end with the three holes is positioned where the gas block will be.
Loosen the screws on the bottom side of the gas block and slide it onto the barrel, and guide the gas tube into the hole in the gas block.
Lightly tighten the gas block screws to keep them in place and align it so that the gas holes in the gas tube and the barrel are perfectly aligned. Then tighten the screws.
Step 10: Install the Gas Tube Roll Pin Through Gas Block and Gas Tube
Install the plug screws at the top of the free float tube handguard
Install the sling swivel studs into the free float tube handguard.
Align the screw holes in the free float tube handguard and the outer receiver nut and screw in the flat head screws.
And that’s it! You can now attach the fully assembled upper receiver assembly with the lower receiver assembly, and you have your AR-15 gun.