Understanding the AR-15 Lower Receiver Parts Kit

The inside of your AR-15 lower receiver may seem like a confusing place. Take a glance and you’ll find it to be jam-packed with a multitude of tiny pieces, all of which need to work together in perfect unison in order for your firearm to function properly. Without some guidance and attention to detail, the idea of building, modifying or repairing the “guts” of a firearm’s operating mechanisms can definitely be intimidating.

Well fear not, fellow gun enthusiasts! Today, we would like remove some of the mystery surrounding the lower parts kit by explaining the simple purposes of all of those tiny pieces. Once you understand your lower parts kit, you’ll be better prepared to tackle your lower receiver installation.

What is a Lower Parts Kit?

The parts needed to install the internal mechanisms of your lower receiver are referred to as a lower parts kit. These inner workings of the lower receiver are made up of approximately thirty pieces, with slight variances dependent primarily upon the type of stock you choose. Although such kits can be purchased from a variety of manufacturers, the overall design and intended function of the contents remains consistent regardless of brand.

What’s in a Lower Parts Kit?

The pile of parts contained in your kit becomes much easier to understand once you can identify each piece. Consult an identification chart such as the one listed at http://blog.cheaperthandirt.com/ar-15-build/ in order to familiarize yourself with your kit.

Standard lower parts kits contain the following:

  • Backplate
  • Bolt Catch
  • Bolt Catch Plunger
  • Bolt Catch Roll Pin
  • Bolt Catch Spring
  • Buffer
  • Buffer Extension
  • Buffer Retainer
  • Buffer Retainer Spring
  • Buffer Spring
  • Castle Nut
  • Disconnector
  • Disconnector Spring
  • Hammer
  • Hammer Pin
  • Hammer Spring
  • Magazine Catch
  • Magazine Catch Button
  • Magazine Catch Spring
  • Pistol Grip
  • Pistol Grip Lock Washer
  • Pistol Grip Screw
  • Pivot Pin
  • Pivot Pin Detent
  • Pivot Pin Detent Spring
  • Safety Selector
  • Selector Detent
  • Selector Detent Spring
  • Stock
  • Takedown Pin
  • Takedown Pin Detent
  • Takedown Pin Detent Spring
  • Trigger
  • Trigger Guard
  • Trigger Guard Roll Pin
  • Trigger Pin
  • Trigger Spring

Grouping the Parts

The simplest way to both organize and understand your lower parts kit is to group the pieces according to their applicable assemblies. Below, you will find a description of each assembly group’s function, as well as the kit parts required for installation.

Trigger Guard Group

The purpose of the trigger guard is to surround and protect the trigger to avoid unintentional discharge of the firearm. This group is comprised of the trigger guard and the trigger guard roll pin.

Magazine Catch Group

The magazine is held in place and released as needed by this section of the firearm, which consists of the magazine catch, magazine catch button, and magazine catch spring.

Bolt Catch

The bolt catch’s job is to stop the bolt from moving forward when the magazine is empty. You will need the bolt catch, bolt catch plunger, bolt catch spring, and bolt catch roll pin for this installation.

Pivot Pin and Takedown Pin

The pivot pin grouping consists of the pivot pin, pivot pin detent and pivot pin detent spring. Because the takedown pin is actually installed with another group, the parts pertaining to its installation will be outlined in the stock assembly grouping. With regards to function, however, the pivot and takedown pins work together.

Removal of the takedown pin allows the upper receiver to separate from the lower receiver. The pivot pin acts as a hinge, allowing access to the inner workings of the upper and lower receivers. These pins also allow easy replacement of the upper receiver, or replacement with an upper that has a different caliber.

Fire Control Group

The fire control group is made up of two subgroups: the trigger assembly and the hammer assembly. These subgroups work together to fire the gun when the trigger is pulled.

The trigger assembly is comprised of the trigger, trigger pin, trigger spring, disconnector and disconnector spring. The hammer assembly parts include the hammer, hammer spring and hammer pin.

When the trigger is depressed, the disconnector moves forward. The hammer catches on the disconnector when the action cycles, and moves to the sear notch when the trigger is released. This prepares the system for firing another round.

Selector Switch and Handgrip Groupings

Although the selector and handgrip have very different functions, the selector switch parts overlap with those of the handgrip assembly. This grouping will utilize the safety selector, selector detent, selector detent spring, pistol grip, pistol grip lock washer and pistol grip screw.

The selector switch allows the user to choose between “safe” and “fire” modes, while the handgrip provides a convenient place to both hold the AR-15 while firing and to manage recoil. Although the pistol grip is the standard part contained in most lower parts kits, many other handgrip options are available for individual purchase.

Buffer/Lower Receiver Extension and Stock Attachment Group

The buffer extension tube and its contents help to absorb recoil while firing. In addition, they push the bolt back into a forward position, allowing the weapon to cycle and seat another round.

The stock attaches to the extension tube and allows the user to position the firearm against his or her shoulder. Stocks are available in a variety of styles, so verify which comes with your kit or purchase an alternate style individually.

The parts required for this grouping are the takedown pin, takedown pin detent spring, takedown pin detent, buffer extension, buffer, buffer spring, retainer, retainer spring and stock. For telescoping stocks, a backplate and castle nut are also needed.

So What Next?

Once you understand the function of the lower parts kit assemblies and have become familiar with the kit components, you are ready to begin your lower receiver build. For detailed instructions on how to tackle the next page of your project, check out our detailed installation directions here.