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80 Lower Receiver Jig - What to Look For When Building It

Posted by Jordan Vinroe on Dec 16th 2019

An 80% lower receiver has been 80% completed. The other 20% will need to be finished, by the gun owner, to be a working firearm. This task will require an 80 lower receiver jig.

Know Your Purpose & Follow Instructions

First, you will have to decide what kind of build you want to complete. Then you can decide which type of  80% lower receiver to buy. This will, then, determine what type of 80 lower receiver jig to use.

Before jumping into this project blindly, familiarize yourself with the entire process by thoroughly reading or watching the instructions included with your jig. If you try and figure it out along the way, you are more likely to make mistakes that can not be reversed. Gather all your supplies together and make sure you have a clear working space. All individual components of an 80 lower jig should be clearly labeled and coordinate with supplied step-by-step instructions.

Types Of Receivers

There is only one correct way to finish an 80% lower receiver - with an 80 lower receiver jig. There are  three types of aluminum receivers and a polymer available:

Billet aluminum - a billet, or milled receiver, is precisely cut from a pure aluminum block by a machinist, to certain specifications. They’re intricately designed and tend to have cleaner lines.

Forged aluminum - a forged receiver is created using a mold and forging two aluminum parts together in heat. The molten aluminum is poured into a mold then the lower is pressed into its final shape using extreme heat and pressure.

Cast aluminum - these lower receivers are not recommended because they typically contain hidden pockets, small cracks, or fissures. When drilling with an 80 lower receiver jig, there is a greater chance to damage the receiver. They are made by pouring A380 aluminum alloy into a mold. With no forging, the lower receiver is then left to harden into its final shape. These defects and imperfections could cause catastrophic failure.

AR-15 carbine rifle trigger and magazine up close

Nylon-Reinforced Polymer - polymer receivers have proven useful and are popular in the AR market. This kind of lower receiver includes a solid-core design to prevent cracking or failure.

With an 80% complete receiver, the safety selector hole, fire control group pocket, and the hammer and trigger pins will be incomplete. They will need to be carefully machined out using an 80 lower receiver jig to become a working firearm. The incomplete lower receiver should only be finished by the person who bought it, using the correct tools, including a jig.

About The Jig

A jig is mandatory for the completion of an 80% lower receiver. It will take care of the measuring and most of the work for you. It serves as a guide with built-in safeguards to prevent user error while milling out the remaining 20% or the receiver. Basically, it shows where and where not to drill and, generally, comes with a top plate and depth guides.

Made with aerospace-grade aluminum alloy, 80 lower receiver jigs can be reused to complete multiple receivers. The jig you use will determine the number of lower receivers that can be finished. All 80 lower jigs allow for three horizontal holes to be drilled into the side of the jig. These are to accommodate the fire/safety selector switch, the trigger pin, and the hammer pin. There are a number of 80 lower jigs to choose from and not all jigs are created equal. They usually fall into two main categories: the drill press jig and the router-based jig.

Drill Press Jig

The drill press jig is the more commonly known 80 lower receiver jig. To finish a lower receiver with this type of jig, the builder is required to use either a handheld drill gun or a drill press. The jig is capable of guiding a builder to drill up to 42 different holes - a “swiss cheese” effect. A drill press lower receiver’s drilling process can start by using a smaller drill bit, like a ⅛” bit, then gradually working up to a larger ⅜” bit. Drilling the “swiss cheese” effect, into the lower receiver, results in eventually carving out the fire control pocket. You will then be able to add your lower parts kit. There are many options for 80 lower receiver jigs. Most will get the job done, but they do vary in quality, durability, and pricing.

Router-Based Jig

The router-based jig is the less common of the two but considered superior by some. The router jig differs from the drill press jig in that it requires two types of machining with less drilling required. Only ten holes are required, as opposed to the 42 or more holes required with a drill press jig. A router and end-mill bit are used with this jig and are helpful in saving time. With this 80 lower receiver jig, a handheld router is used for each of the previously drilled holes. Another 6-11 different passes are made to finish the trigger pocket. An end-mill bit may need to be specially ordered for the router jig system. These 80 lower jigs are generally more expensive, but their strength, durability, and speed make them a popular option despite the price.

Man shooting gun made with 80 lower receiver jig

Why Take On A Project Like This?

The main reason why one would want to go through the trouble of milling and drilling an unfinished receiver is  for the privacy it provides. Completing a frame with an 80 lower receiver jig will ensure you have a firearm that is off-the-books with no serial number or paperwork. It’s also a great way to intimately know your gun.

On A Side Note

It should be noted that building a gun at home requires some gunsmith knowledge and basic comprehension of machining tools. Patience and attention to detail are critical in this process as mistakes and sloppiness may result in a faulty firing weapon, injury, or death. To ensure your safety and the best results possible, always order your DIY gun parts from a  reputable supplier and use quality tools when machining with your 80 lower receiver jig. This will give your homemade gun more of a professional look.