An Intro to AR-15 Lower Parts Kits

Thinking about building an AR-15 at home from an AR-15 lower parts kit or a complete build kit? Make sure you get everything you need and ask for help if you’re not sure. There are a lot of parts that go into one of these fined-tuned sporting rifles. Frankly, there are a lot of parts in an AR that isn’t finely tuned.

Here’s some basic information to get you started.

What Comes with an AR-15 Lower Parts Kits
Many sporting rifles like AR-15s and variants have two main receivers; an upper receiver and a lower receiver, unlike most other modern guns, which have one “receiver” or “frame.”

The lower receiver is the component that contains the FCC (Fire Control Cavity) and accepts the fire selector and trigger assembly. It is the part that is considered by the ATF to be the “gun.”

Some sellers offer what are called AR-15 lower parts kits that come with some of the hardware and parts you’ll need to complete a build. It is important to note that some lower parts kits do not come with a lower receiver blank or even a trigger group.

For example, the AR Lower Build Kit (An AR-15 lower parts kit available at MCS Gearup) contains:

● A mag catch with button and spring

● Safety selector

● Safety spring and detent pin

● Bolt catch assembly (with plunger, spring and roll pin)

● Pivot and takedown pins (plus detents and springs)

● Trigger guard assembly

● Trigger hammer pin

● Buffer tube detent plunger and retainer spring

● Stock

● Buffer tube kit

● Fire controls

To complete a lower, you will need a complete lower build kit and a lower receiver, in addition to a tool known as a jig.

What Is a Jig? Do I Need It?
A jig is a tool that, with a vise, clamps onto a lower receiver blank and serves as a template while cutting out the holes for the trigger, safety, and hammer pins as well as the FCC (fire control cavity).

Jigs are typically made of durable metal like aluminum although some may be made out of synthetic materials or polymers. These are necessary to prevent damage to the receiver blank while machining and also to ensure that your machining is precise.

What Type of Lower Receiver (A.K.A. 80% Lower) Should I Get?
There are two main “categories” of AR-15 lower receivers. These are mil-spec lowers (also known as “forged” lowers) and billet lowers.

Mil-spec 80% lowers are made from 7075-T6 aluminum and are created by forcing hot aluminum into shape (forging it) before machining out the remainder of the contours. They are cheaper to produce and widely available.

Billet receivers are created by extruding a billet of aluminum (commonly 6061-T6 aluminum) before CNC-machining it to its final shape. They are more expensive than mil-spec receivers and available in a wider range of configurations, colors, and finishes.

Neither is necessarily better than the other; it really comes down to a matter of preference.

What Else Do I Need?
In addition to a complete lower parts kit, a lower receiver blank, and a jig kit, you’ll also need a complete upper. You can buy an assembled upper or purchase the parts piecemeal.

A complete upper should come with the following:

● The upper receiver itself

● A barrel

● A handguard

● Charging handle

● Ejection port cover

● Forward assist

● A handguard

● A gas block and tube

Some assembled uppers also come with a muzzle device such as a flash hider or a muzzle brake. You’ll also need a bolt carrier group, and, if the AR-15 lower parts kit did not come with them, a pistol grip, a magazine, and a trigger assembly.

Learn More at MCS Gearup
Interested in learning more about the parts that come with an AR-15 lower parts kit? Want to pick up a high-quality kit yourself and get started on your own home-build project?

Visit MCS Gearup, either online at or by phone at 239-848-6757. They have high-quality upper and lower receiver parts you need to get started on your project and can help field any of your questions as well.

For more information about Pistol Braces and Assembled Uppers Please visit: MCS Gearup.

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