How to Mill Out An AR-15 80% Lower Receiver
If you’ve done any research into 80% AR-15 lowers, you probably already know some of the benefits that they offer: no background checks, no transfer fees or NICS fees, and no unnecessary money spent to recoup a manufacturer’s branding and marketing costs – not to mention the satisfaction of knowing that you built your lower with your own two hands. If you’re nervous about milling your own lower for the first time, that’s understandable – it can certainly seem like a daunting and highly technical task. But the good news is that completing your own 80% lower is actually quite simple if you’ve got the right tools!
We’re going to walk you through what you’ll need, explain the actual process of completing your lower, and share some tips on how to avoid common mistakes along the way.
Tools You’ll Need
- Drill Press or Router + Power Drill
- 80% Jig and Bits
- Cutting Fluid
- Ear and Eye Protection
- Shop Vacuum
- Calipers (Optional)
The actual process of milling your receiver is going to depend on whether you choose to use a drill press or a router (a milling machine is another option, and arguably the most effective one, but unless you’ve got access to a seriously well-equipped machinist shop or plan on producing a whole bunch of lowers, it’s not a very practical choice from a financial perspective).
Generally speaking, a drill press is going to be more stable and accurate than a router, but using a router is significantly faster, and a router setup is usually cheaper and much more portable. There are plenty of fans of both methods, and which one you decide to use will largely come down to your budget, how much room you have to work, and how much patience you have.
Regardless of which path you choose, the basic tools you’ll need will be mostly the same. Aside from the 80% lower itself and proper safety equipment, the most important thing you’ll need is the 80% jig and the proper bits. The jig for a drill press or mill is slightly different from a router jig, so make certain that you’re buying the right one.
Here is the basic process for each method:
If you’re using a drill press or mill, you’ll start out by placing the 80% lower into the jig and securing it in the vise, following the provided instructions based on which company you buy your jig from.
The next step is to install the pilot hole template and drill out the receiver pilot holes with a 3/8” bit. Use plenty of cutting fluid, such as WD-40, during this step to improve the performance and life of your drill bit.
Once that is complete, it’s time to replace the pilot hole template with the milling template and make your next decision: to mill the receiver with the plunge-cutting technique, or using a lateral milling technique.
Plunge-cutting involves switching to the short 3/8” end mill bit (if you want to ensure proper precision, use either a stop collar or verify cutting depth with calipers) and using it to drill vertical holes until the fire control cavity is hollowed out, after which any rough spots or burs can be polished and lightly milled away.
Lateral milling involves using the drill press to cut side to side, milling away thin layers of aluminum with each pass. While a drill press is typically not designed to hold up to very much lateral load or pressure, aluminum is soft enough that many people choose to use this method, as it is can be quite a bit faster. Just be sure that you’re using proper safety equipment, and do multiple shallow cutting passes to avoid breaking the bit.
With both methods, you will be making two passes: the first will mill the receiver to a depth of 1.375 inches, and the second will mill the receiver to 2 inches deep.
Once that is complete, the final step is to install the last cutting template, usually marked “5/16 EM THRU BOTTOM.” Switch to the long 5/16” end mill bit and use the template to cut out the trigger slot. Without removing the template, remove the entire jig from the vise and reposition it horizontally with the right side facing up, then use the guide holes in the side of the jig to drill straight through the receiver for the hammer, trigger, and safety pin holes.
With that finished, you now have a complete stripped AR-15 lower ready for a parts kit install!
Router + Power Drill
If you’re using a router and power drill instead of a drill press, the process is almost exactly the same, except you’ll be using a slightly different jig and you’ll only need to drill a single pilot hole to begin the milling process. The router jig will come will come with several sets of guide pins to help set the proper cutting depth, and you will make multiple milling passes until the proper depth for each set of guide pins has been reached, making sure to move in slow, circular motions while routing.
As with the drill press method, you should be using your cutting fluid to lubricate between each milling pass – this keeps the router running smoothly, and is critical to avoid damaging your tools or your lower.
Once the milling is complete, you only thing left is to use the power drill to drill out the fire selector and pin holes. Congratulations, you are now the proud owner of a 100% AR lower!
Mistakes to Avoid
While milling an 80% lower isn’t as complicated as it looks, there are still a few important mistakes to avoid during the process. The biggest potential mistake is drilling or milling the receiver to the wrong depth – remember the old adage, “measure twice, cut once,” and double check your measurements each time you make a pass – this is where having good calipers and stop collars comes in handy.
The other big mistake is rushing – it’s easy to accidentally throw off your alignment by over-tightening a vise, leaving a guide plate too loose, or just being sloppy with a power drill. Take your time and go easy – saving a few minutes isn’t worth the risk of ruining your lower and having to wait for a new one to arrive.
Hopefully this guide has given you the knowledge and confidence you need to give milling your own 80% lower a try – it can be a very rewarding experience, and it’s a great skill to learn if you plan on building multiple rifles.