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4 Reasons to Use an 80 Lower Jig When Completing an 80 Lower


Lately, we’ve discussed why you need the right tools to complete an 80% lower precisely, lest you end up with an unreliable – even dangerous – rifle or pistol. So, of all the gunsmith’s tools, why is the 80 lower jig the most important of them all? Let’s dive into the critical reasons why you should use an 80% lower jig when completing an 80% lower:


#1: An 80% Lower Jig guaranteed to work!

Yes, even veteran machinists make faults in their calculations, drill depths, tool sizing, and passes on the mill. Completing an 80% lower using a mill takes true grit, a lot of time, and many measurements and calculations – many of them prone to misjudgment without a digital readout.

The venerable 80% lower jig, however, does all the work for you.


It provides the proper drill depths and milling depths, it guides you along your machining surface, and, if used correctly, it all but guarantees your finished AR-15 lower receiver is machined to exact specifications. It’ll be ready for any parts kit and it’ll shoot reliably.


#2: The 80% Lower Jig is faster!

Face it: Trying to machine a precision-manufactured piece of metal without the use of pre-measurements and a custom-built vise will require hours of research, calculating, and re-measuring. The fastest 80% lower jig, the new Modulus Arms Router Jig Extreme, allows you to machine an 80% lower in 15 minutes flat. There is simply no competition nor argument that can be made against the 80% lower jig if discussing time and convenience.


#3: The 80% Lower Jig is safer

Safety should be your number one concern when using power tools and manipulating metal. Although it is technically simple to machine an 80% lower, there is plenty of risk present when not using equipment designed to prevent mishaps.


Our 80% Lower Jigs all utilize built-in drill and mill depth guides, and they feature roll pin systems with hardened side plates to prevent your bits from touching anything but your lower’s aluminum. In a worst-case scenario, your 80% jig’s plates and guide system will help contain accidental breakage or tool failure, reducing the risk of injury.


#4: The 80% jig is more affordable

The 80% lower jig is quite literally a miniature machine shop in a single tool. You get your vise, your mill, your drill guides, and your workstation, all in one. Machining an 80% lower without a jig will likely be more expensive: You’ll need to invest in more vises and fabricate your own jig, or work in a pre-defined and measured space, calculating the X, Y, and Z axes of your drilling and machining points.


That will require measuring equipment. You will also need to purchase your own end mill bits, transfer punches, drill bits, a drill press or handheld drill stabilizer, and much more. Trust us, the 80% lower jig is much, much more affordable.

What’s more, most of our 80% jigs provide all the drill bits and end mill bits you need! The cost savings on tooling alone will make your 80% jig worth the investment on the first build – never mind your future builds.



DISCLAIMER: If you are new to the world of DIY AR-15 building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. We are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.

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