A Comprehensive AR-15 Parts List

Building your own AR-15 can be a rewarding project if you do your homework and know what you want before you begin. There are more than 50 AR-15 parts to collect separately before you start building. Buying components separately or in kits allows you to customize your rifle to your own unique needs and preferences. This will allow you to build your own rifle from scratch, upgrade an existing rifle, or stay compliant with state and/or Federal laws.

Lower Receiver

The lower receiver is the part of the rifle that is engraved with the serial number typically issued by the manufacturer. When purchasing a stripped or complete lower receiver, the serial number will already be engraved. If building from an 80 percent lower, which doesn’t require the use of a FFL, the serial number is optional in most states. . Complete and stripped lowers must be shipped to an FFL dealer and you must pass a background check in order to pick it up. The lower receiver will serve as a point of attachment for the pistol grip, the trigger, magazine, and the buttstock.

Lower Parts

There are 31 separate parts that are housed in the lower receiver. This includes the pistol grip, trigger, trigger guard, springs, bolt catch, pivot and takedown pins, selector switch and hammer. You can purchase all of these parts separately, but if you don’t need any upgrades it may be more expedient to purchase a lower parts kit. You may also choose to get a lower parts kit that does not include the trigger so that you can purchase that separately.


The AR-15 trigger is one item that many people like to customize. A single-stage trigger with a light pull weight is often chosen for stable shooting situations where accuracy is of primary concern. A two-stage trigger requires a longer pull when firing. The first stage is usually the heavier part of the pull and it ends at the beginning of the second stage which feels like a crisp, sharp pull which cause the weapon to fire.

The primary difference between single- and two-stage triggers is that if you release a two-stage without firing, its’ spring will push it back to its original position. This is the choice of most military weapons, law enforcement, and competition. You should consider your shooting style and situation when selecting trigger type and pull weight. Of course you could assemble your fire control group yourself, which would include hammer and hammer spring, trigger and trigger spring, disconnector and disconnector spring and two fire control pins.

Pistol Grip

An upgraded pistol grip is also something you may want to consider. They can be curved with finger ridges for better grip or patterned to add distinction to your rifle.

Selectors and Catches

The selector is the safety switch. The bolt catch (or bolt release) stops the bolt from going forward when the magazine is empty or when you want to inspect the chamber. The magazine release drops the magazine or drum from the weapon.

AR-15 Stock

The buttstock will attach to the rear of the lower receiver and is another area that should be customized to the user. A buttstock can be either collapsible or fixed and can come in a variety of styles. They are often fitted with an extra butt-pad to reduce recoil impact and improve shoulder grip.

Buffer Tube, Buffer, and Buffer Spring

The top portion of the stock will hold the buffer tube. Inside that will be the buffer and buffer spring which are essential components of the AR’s firing mechanism. The buffer is the point of contact for the bolt carrier as it moves rearward into the buffer tube. As the buffer spring recoils, it pushes the buffer (and bolt carrier) forward, loading the next round of ammunition and closing the bolt for the next shot. There are two different sizes of buffers – rifle length and carbine length. Rifle length buffers are used in fixed-stock weapons to fill the void in the longer buffer tube. Carbine size buffers should be used in collapsible stocks. These are not interchangeable.

Upper Receiver

This group contains the forward assist, the charging handle and the ejection port. It houses the bolt carrier group and is the point of attachment for the barrel and gas tube. The charging handle is what you use to chamber your first round. It catches the bolt carrier as you pull the handle rearward leaving space in the chamber for a round to be fed from the magazine. The forward assist pushes the bolt carrier forward ensuring that the bolt is locked in place. The ejection port allows for the expulsion of spent shell casings making room in the chamber for the next round.

Bolt Carrier Group

This is arguably the most important set of AR-15 parts in the entire rifle. The bolt carrier group includes the bolt carrier, firing pin, firing pin retaining pin, bolt, bolt cam pin, extractor, extractor pin, and extractor spring & insert. The firing pin moves forward, crushing the primer on a bullet and igniting it. This ignites the gunpowder, generating gas pressure and forcing the bullet forward. The firing pin is housed inside the bolt which presents the firing pin to the bullet at the correct position and distance. The bolt carrier holds the bolt and includes a gas key. This gas key will meet with the gas tube so the muzzle gasses can be received forcing the entire bolt carrier group rearward.


The barrel of the weapon is the part that the bullet travels through on its way to the destination. The come in a variety of lengths, weights, calibers, and twist patterns. You will also need a barrel nut to attach the barrel, and a hand guard of some type. There are many handguards that allow for the attachment of extra devices – i.e. lights, scopes, etc. You may also want to consider some type of muzzle device. This could be a flash suppressor or compensator and can be useful in dissipating muzzle gasses.

Gas Tube and Gas Block

The gas tube forces muzzle gasses back toward the upward receiver. This pushes the bolt carrier backward initiating the next firing cycle. The gas block channels gasses from the muzzle to the gas tube. This is a very forceful process and is the cause of rifle recoil. There are adjustable gas blocks available that can allow you to fine-tune your gas system. If minimizing recoil is a consideration, an adjustable block may be worth the extra investment. Whichever gas block you choose, you should select a gas tube that is compatible with your barrel length.


Magazines are available in a variety of sizes including 10 round and 30 round flat magazine or 50-100 round drums.


There are a variety of AR-15 parts that are not essential to the functioning of the weapon, but will greatly improve its effectiveness. At a bare minimum, you will want to include sights, a sling, and a cleaning kit. Iron sights can be mounted both on the barrel and the upper receiver. Scopes serve the same function as a sight but can improve accuracy. Lasers allow pinpoint accuracy, but may not be suitable in all weather conditions. Lights on handguards and magazine holders on a buttstock can reduce the number of different things you need to carry. A tool kit that includes a vice block, site adjustment tool, and wrench assists or armorer’s multi-tool for removing barrel nuts, muzzle devices, and buttstocks.