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A highly modular and customizable rifle platform, the AR-15 is the gun builder or budding gunsmith’s dream weapon. There are several reasons gun owners build their own rifles. But where should you begin?
Why Build an AR-15 Yourself?
Dozens of companies manufacture complete rifles, parts, and assemblies, allowing you to choose the project difficulty or involvement you’re prepared to undertake.
Many new shooters and those unfamiliar with the AR-15’s intricacies purchase a factory-assembled rifle. Others choose to build one from the ground up. There is a multitude of reasons for this, which include:
By selecting affordably priced parts and assembling them yourself, you can build a budget-friendly alternative to a factory-assembled rifle.
One of the advantages of the AR-15 platform is its degree of customizability, allowing you to truly personalize your rifle for form and function.
Whether you’re a firearms enthusiast or intend to enter the gunsmithing profession, building an AR-15 is a highly educational experience and can add to your skillset.
There’s nothing quite like the satisfaction of building a rifle yourself, especially if you decide to mill your own 80% lower.
Before asking how to build an AR-15, you should decide what kind of weapon you want to assemble: pistol, carbine, or rifle.
You can start by choosing a barrel. Piston-length barrels are typically between 7 and 11”. Carbine-length barrels typically vary from 14.5” (M4/M4A1), with a pinned and welded flash suppressor, 16” (non-NFA minimum), and 18”. Rifle-length barrels usually fall between 20 and 24”.
The lower receiver is legally the firearm, according to the ATF. It houses the fire control group, contains the magazine well, and bears identifying information regarding the weapon (i.e., make, model, caliber, and serial number). Lower receivers are available following conditions:
1. 80%/Receiver blank
This is an incomplete receiver or receiver blank. As a result, an 80% lower receiver does not qualify as a firearm. An 80% lower does not possess the pocket for the fire control group. The magazine well, however, is usually broached or milled at the factory.
You should buy an 80% lower if you want to finish the necessary machining operations yourself, thereby building the rifle in the truest sense. You’ll need a milling machine and a drill press.
A stripped lower receiver is a finished part and, thus, the legal equivalent of a firearm. However, while machined to completion, a stripped receiver is strictly the aluminum housing—there are no additional parts.
This is the type of lower receiver you should buy if you want to assemble the component parts yourself but don’t have access to the necessary machine tools (e.g., a mill) or possess the requisite knowledge to operate them.
A complete lower receiver is the housing + the lower parts kit (LPK). No further assembly is necessary.
There are several different types of upper receivers to choose from. The classic A1 and A2 upper receivers have an integral carrying handle, which houses the rear sight assembly. In the A2 variant, the rear sight is adjustable for both windage and elevation.
Modern variants have a flat top with a MIL-STD-1913 Picatinny rail for mounting optics. In the A1 and A2 variants, an adapter is necessary to attach a rifle scope.
Gas System and Length
In an AR-15 that uses the Stoner gas system, what is typically called “direct impingement,” there are four types of gas system lengths you should consider, depending on the barrel.
1. Pistol length
The pistol-length gas system is for barrels of less than 10”. The distance from the gas port to the receiver is four inches.
2. Carbine length
The carbine-length gas system is for barrels 10—18”. Port distance is 7”.
The mid-length gas system is for 14–20” barrels. Port distance is 9”.
4. Rifle length
Finally, if you’re an AR-15 with a barrel 20” or longer, there’s the rifle-length gas system. Port distance is 12”.
Different rifles require different buffer assemblies. The buffer, containing either steel or tungsten, adds weight to control the bolt velocity during the cycle of operation. The weight of the buffer that you need depends on several factors, such as the gas pressure and caliber.
Once you’ve determined what kind of AR-15 that you’d like to build and selected the necessary parts, you can begin the assembly. The assembly process detailed below is not exhaustive but includes the most complicated parts. When researching how to build an AR-15, these are the areas with the highest learning curve.
Placing your upper receiver in a vice block, insert the barrel into the front of the upper receiver, ensuring that the peg is aligned with the notch in the threads. Slide the barrel nut over the barrel and tighten it on the receiver threads by hand. Use an armorer’s tool and torque wrench to tighten the barrel nut the rest of the way.
You should be applying 30 ft-lbs. of torque. You must ensure that the hole in the receiver for the gas tube aligns with the hole or nut in the barrel nut.
Gas Block and Tube
The gas tube feeds propellant gases from the barrel into the action to facilitate the cycle of operation. It has two horizontal holes—one for the retaining pin and the other for the gas port. Align the gas port in the tube with the gas port in the gas block.
Measure the depth and rotation of the gas tube on the gas block using a pen or pencil, insert the gas tube into the gas block, and tap the roll pin through the hole in the gas block to hold it in place. The best way to test the alignment of the gas port holes is with an air compressor.
If you’re using a low-profile gas block that’s held in place using vertical screws, you may decide to dimple the barrel using a drill press. Alternatively, if you’re installing a traditional AR-15 gas block and the barrel has already been drilled, insert the two barrel pins and tap them into position.
Headspace is the distance between the point in the chamber that stops forward movement and the breech face. The headspacing point controls the seating depth. You can check the headspace by using go and no-go gauges.
When you insert the go gauge into the chamber, the bolt should enter the barrel extension, rotate, and lock. However, the no-go gauge should prevent the bolt from rotating and entering battery.
The Last Word
The process of building your own AR-15 can be a highly rewarding experience. In addition to saving money, it also lets you truly personalize your rifle.