Building your own AR-15 from scratch is as exciting as it can be intimidating. Builders have many decisions to make. One of those decisions, and arguably the most important decision, is what lower receiver to use.
The “heart” of the rifle is contained in the AR-15 lower receiver and is the only serialized part of the rifle. Certain calibers, such as the .300 Blackout, .223/5.56, 7.62×39, require your lower to be the 5.56 M4 standard design. If you want to build a larger caliber like a .308 then you will need to have a bigger AR-10 style lower receiver.
Once you know how much money you want to spend on your AR-15 build and have determined which caliber you want to go with, you’ll need to decide whether you want to build with a complete lower, stripped lower, or 80% lower.
Stripped vs Complete vs 80% lower
In general, most AR-15 lower receivers are going to function the same. The AR-15 lower receiver is comprised of the lower parts kit. The lower parts kit typically includes the pistol grip, butt stock, buffer, buffer spring, trigger assembly (or fire control group), in addition to the magazine housing. The lower receiver also provides the pivot and takedown pin connection point for attaching the upper receiver.
For the most part, builders can purchase their lower receiver in three different forms, complete lower, stripped lower, and 80% lower. A complete lower has all the needed components already installed in it. All you have to do once you receive it is install the upper receiver. A stripped lower receiver, on the other hand, requires you to install the lower parts kit, buffer assembly, butt stock, and other necessary parts. For the more adventurous, and for those that don’t want to register your AR-15, an 80% lower is the best option.
More About 80% Lower Receivers
An 80% lower is a lower receiver that requires you to finish milling the last 20% in order for it to be a functional lower. Out of the three types of lower receivers, the 80% lower is the only one that the ATF doesn’t require the use of an FFL and doesn’t require registration.
Keep in mind that even though the ATF says you don’t need to register an 80% lower, your local city, county, and state regulations may require you to register (talking about you, California). Most jurisdictions have no laws or minimal laws in that regard, but do some digging if you are unsure.
Cast, Forged, or Billet
As you are researching 80% lowers, you’ll likely come across three manufacturing methods of the lowers. Cast, forged, and billet are the top three methods used and differ in reliability and looks.
A cast 80% lower is pretty much what it sounds like. A mold is created the provides the shape of the lower receiver. Aluminum gets melted down and poured into the mold. Once the aluminum has cooled enough to solidify, the lower is taken out of the cast and is machined to finish the receiver.
Cast lower receivers provide a lower price point for builders but are typically a weaker lower receiver when compared to forged and billet. The overall life expectancy of a cast 80% lower receiver isn’t near that of a forged or billet lower.
Think of a forged 80% lower as being pounded into its desired shape. The forged lower gets hammered into forging dies until a general shape is made and is then removed from the die and machined to it’s final shape.
A billet 80% lower differs in that aluminum ‘bars’ are made by compressing the aluminum between rollers. Once the aluminum bar is compressed to the desired density, it is machined or cut to the shape of an 80% lower. Billet receivers are a common choice for AR builders that are focused on aesthetics. This is because the billeting process provides an overall better look. With that, though, comes a much higher price tag.
Which Is Best?
Without sounding to cliche, the best 80% lower receiver depends on many factors – price, look, and reliability are just a few determining factors. The most popular choice among builders and manufacturers alike are forged lowers. They are priced competitively, last a long time, and provide builders a great option when building an AR-15.