What 80 Percent Lower’s Right For You?

We’ve covered the reasons why any new gun owner should consider buying an 80% lower for their next pistol or long gun build. It provides plenty of unique advantages that include lower taxes and costs, fewer fees paid to an FFL, and the avoidance of paying a premium for a branded firearm. Most of all, you get to build it entirely on your own.

But that leaves you with the most important question of all: Which 80 percent lower is right for you? There are a plethora of ‘em available across the web, but you should only consider a quality unit from a reputable seller. More importantly, you need to consider all the different factors and features that’ll dictate how, exactly, you fabricate your new firearm:

Material and finish

Some 80% units are made from polymer, while others are made from aluminum – either forged or billet. These aluminum units are heat-treated and are tough as Hell, which can make fabrication a tad more difficult. If you’re inexperienced with machining metal, a polymer receiver or frame is a great first project, since these can typically be fabricated with hand tools. If your unit comes without a coating or finish, you can apply your own for a customized piece.


Your receiver or frame will often ship with engravings to indicate the safety lever’s FIRE and SAFE position. Some units ship un-engraved, allowing for you to add your own markings or custom roll mark or stamp. In states that require serialization, you may need to visit an engraver to have a unique, identifying number applied to your new firearm. This is applicable in California and other states.

Commercial vs. Mil-Spec

The AR-15 is the weapon most 80% builders choose to fabricate. The modern AR has to standards of fitment: Mil-spec, which requires the use of a forged 80% lower, or commercial, which requires a billet receiver. These two standards use different threads and fittings for the buffer tube (the component that secures your buttstock or pistol brace). 

Caliber and Configuration

80% lowers are no longer reserved for the old-school AR-15 rifle chambered in just .223 and 5.56 NATO. These days, the AR platform comprises loads of available configurations that include rifle, pistol and SBR builds. Calibers range from .22 LR, through the various hunting cartridges, specialty rounds like .300 Blackout and 6.5 Grendel, up to big long-range loads like 6.5 Creedmoor and .308.

And of course, the handgun market gets some love these days. 80 percent frames can now be sourced to fabricate a custom GLOCK-compatible pistol, a lightweight aluminum 1911, or a modular Sig Sauer P320. Your choice of caliber and configuration should come first, with the lower or frame coming second.

Types of 80% lowers and Frames

Let’s take a look at the top 80 percent lower and frame options available and explain what their configurations and specs entail.

Forged and Billet AR-15 Lowers

These two units comprise the bulk of most 80% builds. They’re made for the original AR-15 configuration, which is to say they both chamber .223 Remington or 5.56 NATO. Builders who opt for a custom rifle or pistol in a specialty cartridge can instead opt for the 6.5 Grendel, for long-range shooting, or 300 Blackout, made for using a suppressor. The 300 BLK and Grendel loads are perfect for short-range nighttime and long-range hunting, respectively.

Billet AR9 Lower Receiver

The AR9 lower is designed for 9mm AR builds, mostly configured as pistols. These receivers can accommodate any 9mm AR upper, including a rifle-length upper for maximum velocity and accuracy at range. The proprietary design of the AR9 80% lower negates the need to modify your existing AR-15 receiver to accept 9mm magazines. This lower uses any Glock magazines and comes with its own ejector and magazine release to be installed after fabrication. No gas system is required for this build; the 9mm cartridge relies on blowback operation with a modified bolt.

.308 (AR-10, LR-308) Lower Receiver

Based on ArmaLite’s old AR-10 design, the modern LR-308 receiver is the .308 Winchester / 7.62×51 NATO variant of the AR-15. Introduced before the AR-15 itself, the AR-10 was the battle rifle of choice in testing for a new service rifle. The AR-10 has been replaced by DPMS Panther Arms’ take on the platform, officially called the LR-308. It provides comparable performance in a package that is much more affordable, with greater access to custom components. Assembly and fabrication are identical to that of the average AR-15.

80% 1911 Frame

This aluminum 80% frame, introduced by Stealth Arms, provides a massive and easy-to-fabricate upgraded frame compared to the original 1911’s steel frame. It weighs approximately 50% less once fabricated, allowing for a more ergonomic and easy-to-control shooter. Frames are available with a custom Hardcoat anodized finish, or a raw finish so you can apply your own coating or paint job. These frames can be configured as a Government (full-sized, 5”) or Commander (compact, 4.25”) handgun. These frames are compatible with all retail 1911 parts.

GLOCK-Compatible P80 Frame

The P80 frames earned favor among builders as an affordable alternative to buying a GLOCK handgun. Compatible with all Gen3 components for virtually all GLOCK series and models, the P80 frame is also one of the easiest-to-fabricate frames available. It requires just basic hand tools and light fitting and sanding before assembly. These units use a disposable jig, which comes with the necessary bits for fabrication. A hand drill, Dremel, and files can be used, without the need for more complex equipment.

Sig Sauer P320 80% Fire Control Unit

The Sig P320 Fire Control Unit is the modular “heart” of the P320 handgun. It serves as a universal host for a plethora of calibers and configurations, ranging from subcompact to full-size and provides four calibers. The FCU is the only part of the Sig P320 that’s considered the firearm, in similar fashion to the 80% lower and AR-15 platform. Fabrication requires a Dremel, files, and light sanding and fitting. Once fabricated, the FCU is compatible with Sig P320 parts.

Frequent Questions

Q: How do I fabricate an 80% lower?

A: You will need a tool kit called an 80% jig. The jig is a proprietary milling machine that requires the use of a drill press or mini-mill. The jig shows the end-user where to cut and drill the receiver, transforming it from a receiver blank (not a firearm) into a functional stripped lower receiver, a firearm under federal law.

Q: Are these units banned in some states?

A: Yes. Some states, like New York and New Jersey, have outright banned 80% lowers. Other states, like California, require serialization of the receiver before fabrication. Although these firearms are not serialized by default, their ownership means you must still adhere to all gun laws in your state. That may include serialization even still, or requiring that your handgun or long gun be locked in an appropriate gun safe. The end-user must also submit to a state background check to verify eligibility of firearm ownership. If you’re unsure of the legality of fabricating or otherwise owning a firearm in your state, you must consult legal counsel beforehand.

Q: Are these units, once finished, any different from a retail firearm?

A: Functionally, no. Any AR-type rifle or handgun fabricated from an 80% unit is identical in form and function to its branded retail counterpart. This allows for easy assembly and further customization by way of purchasing your own components piecemeal, minus the premium costs associated with buying a fabricated rifle from an FFL or gun dealer online.