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The AR-15 comprises two receiver assemblies — upper and lower — connected by a front pivot pin and a rear takedown pin. Each assembly contributes to the reliable function of the gun and your ability to shoot it accurately.
Assembling Your AR-15
Whether you're assembling an AR-15 from the ground up or simply swapping parts, you should ensure that your lower receiver assembly is ready and capable of meeting a standard of quality and performance that corresponds to your needs.
While the barrel, bolt carrier group, and gas system are essential to your rifle’s operation, the lower receiver assembly is the part of the weapon with which you interact the most.
You’re manipulating the bolt catch, magazine catch, selector lever, and trigger constantly throughout your operation of the weapon. If these parts don’t work with you, your shooting experience will likely suffer.
Lower Receiver Assembly Checklist
The AR-15 lower assembly consists of multiple parts that allow you to fire the gun. The lower receiver is the machined, cast, or forged aluminum housing that contains the fire control group, magazine catch, and bolt catch. It also provides attachment points for the pistol grip and receiver extension/buffer tube. It also provides the magazine well — the rectangular slot into which you insert the magazine.
A complete lower receiver, a receiver blank either you or the manufacturer has machined to completion, is legally the firearm.
You’ll need to have a checklist of parts to ensure your lower receiver meets standards for quality and usability.
If you only want to build a rifle for recreational target shooting, the standards can be lower. A bare-bones mil-spec AR-15 is sufficient for these purposes. It will also perform reasonably well regarding reliability, provided that you use high-quality magazines and factory-loaded ammunition in good condition.
However, if you need a workhorse that is also accurate, ergonomic, and easy to shoot, you should consider your parts and assemblies carefully.
Fire Control Group
The fire control group is also called the trigger assembly or trigger mechanism and plays a vital role in the function of your weapon. You can settle for a mil-spec fire control group, which is functional; however, many shooters find this option inadequate.
Mil-spec triggers are often heavy and, due to machining roughness and a lack of polish, tend to be gritty or creepy. Shooters use grit to describe inconsistencies during the pre-travel phase once the trigger hits the wall — the point at which pressing the trigger meets additional resistance from the sear and mainspring.
You can perform a DIY trigger job if you follow specific guidelines and buy the correct tools. However, many gun owners buy a drop-in replacement trigger assembly or hire a gunsmith to perform this service.
Fortunately, a variety of companies offer competition-grade trigger assemblies to improve upon the military standard. These assemblies provide a light, smooth, and crisp trigger pull, a short reset, and adjustability using a screw.
The pistol grip is sometimes overlooked as a part of the AR-15 lower assembly. However, in addition to allowing you to hold the weapon comfortably when firing, the pistol grip retains the detent pin for the selector lever. As a result, you should ensure that the pistol grip you choose meets your standards for ergonomics and function.
Some shooters find the mil-spec or A2 pistol grip inadequate. If you want a pistol grip that fills your hand more comfortably or has a spare parts compartment — for carrying extra cartridges, batteries, or a cleaning tool — there are various options available from such manufacturers as Magpul and BCM.
Bolt Catch, Magazine Catch, and Selector
The bolt catch is the part that locks the bolt open when the last round is fired, raised by the magazine follower. When you drop the empty magazine and insert a fresh one, you depress the bolt catch with your left thumb or strike it with the palm of your hand. This releases the bolt, allowing the action spring to drive it forward, chambering a round.
The magazine catch, also called the magazine release, is a button on the right side of the receiver. Depressing this button, which falls under the index finger of your right hand, disengages the magazine and allows it to drop or you to remove it.
The selector lever is a rotary lever on the left side of the lower receiver, which allows you to choose the firing mode. In civilian-legal — non-NFA — AR-15-pattern firearms, the selector lever has two modes: “Safe” and “Fire.” In NFA weapons, you may also have a “Full Auto” or “Burst” setting.
The first consideration you may have regarding these parts is ambidextrous functionality. The standard AR-15 design is specifically intended for right-handed soldiers and shooters.
If you’re left-handed, you can buy various ambidextrous or bilateral options for all three parts, increasing their accessibility. If you plan to share your rifle with left-handed friends or family members or need dual-shoulder access for competition shooting, ambidextrous controls can be highly beneficial.
You can also find oversized and textured bolt and magazine catches to increase your ability to engage these controls under adverse weather conditions, when your hands are wet, etc.
As for the selector lever, you can also determine the appropriate “throw,” which refers to the rotation necessary to engage and disengage the safety. The standard selector lever throw is 90°. However, you can also find selectors that rotate between 45 and 60°, if you’d prefer a shorter throw to reduce the time interval between taking the safety off and firing a shot.
Buffer Spring/Action Spring
The buffer assembly consists of a series of weights designed to control the velocity of the bolt carrier assembly and, thus, the cyclic rate during the cycle of operation. The buffer weight and action spring length you’ll need will depend on the barrel length, gas system length, cartridge, and other factors related to your specific rifle build.
Generally speaking, shorter barrels, shorter gas system lengths, and more powerful calibers — e.g., .458 SOCOM — require heavier buffers to resist the increased bolt thrust and bolt velocity.
An incorrect buffer and action spring combination can compromise the reliability of the system and accelerate wear.
Tools and Accessories
Don’t skimp on the correct tools for assembling your AR-15. For the upper receiver, you’ll need a bench vice, vice blocks to protect the parts you need to hold, and other equipment, such as an armorer’s tool and pin punches.
As for accessories, don’t neglect high-quality magazines. While not a lower-receiver part, the magazines that you choose should not be cheap junk. A poor-quality magazine can be a liability, especially in a defensive rifle that must perform with 100% reliability.
When you’re building an AR-15, the lower receiver assembly is as important to reliable operation and usability as the upper. As a result, you must select your parts carefully, paying attention to the quality of craftsmanship and ergonomics.