​How to Build An 80% Lower: Beginners Guide thumbnail image

​How to Build An 80% Lower: Beginners Guide

80 Percent Arms   |   Aug 12th 2020

According to the Second Amendment, all U.S. citizens hold the right to bear arms. But since the founding of the Constitution, we’ve witnessed countless caveats come into play, along with immense difficulty in purchasing or selling firearms throughout the country. That’s why, to this day, 80 percent lowers have experienced a boom in popularity. You’re able to purchase an unfinished lower and complete the build yourself, thereby saving you time and money while avoiding legal hassle.

But how do you build an 80% lower? It’s a question that all newcomers ask when they attempt their first rifle build.

Tools Required to Build An 80% Lower

For most jobs, your hands are the best tools you have. But for completing an 80 percent lower AR build, you’ll need a few pieces of machinery on hand.

  • Drill press or milling machine
  • Vise
  • Machining oil

While optional, it’s recommended you keep the following nearby:

  • Digital calipers
  • Drill stop collars

Furthermore, you’ll need a few end mill bits and drill bits but, depending on the jig you purchase, they are occasionally included in the packaging.

  • (1) 3/8 Drill Bit - 5”
  • (1) 5/32 Drill Bit - 3”
  • (1) 3/8 Short End Mill - 3”
  • (1) 5/16 Short End Mill - 4”
  • (1) 3/8 Long End Mill - 4”

Step-by-Step Guide to Build An 80% Lower

With the tools you need on-hand, it’s time to tackle your first 80% lower build. Don't overthink it, we've made it very easy. Take your time, follow the instructions, and you're guaranteed a great result.

Step 1: Secure the Lower

The first step in completing your new AR build is to secure the lower in the right side jig plate. The pins on the plate should rest inside the rear take-down of the AR lower and from the pivot pinholes. Using the left jig plate, secure the opposite side and stand the lower up vertically.

Step 2: Install Template Plate Holder

With the jig secure, it’s time to install the template plate holder. You will seat the holder on top of the jig where the rounded edge rests along the buffer tube housing. Bolt the plate holder to the jig side plates using the two biggest hex-head bolts included with the setup.

Step 3: Install Pilot Hole Template

For step three, you’ll now install the pilot hole drilling template using the two included countersunk hex-head bolts in the jig package. The first template is typically marked with “3/8 DR 2.00 Deep From This Face.” Next, tighten the jig and lower it into your vise for further machining.

Remember: To avoid misalignment, do not over-tighten the vise. Apply light pressure with the clamp to ensure the jig does not move during the process.

Step 4: Install 3/8” Drill Bit

The jig package included a 3/8” drill bit, which you’ll now want to install on your drill press or milling machine. Align both the jig and the lower with the tooling, then drill through the pilot hole guide on the top template plate. You’ll want to drill precisely 2.000” deep from the top. Therefore, be sure to set your drill or mill depth appropriately.

Step 5: Drill Receiver Pilot Holes

We mentioned you might want machining oil nearby for lubrication. Here’s where you’ll use it. Lubricate the template plate guides and receiver using your machining oil to protect your drill bits. You’ll require this bit for the safety lever pinhole.

Now, drill down 2.000” deep into the receiver from the face of the top template plate. You may wish to use a stop collar and calipers to verify the depth, as it must be exact. Once the pilot holes are drilled, remove the 3/8” bit and pilot hole plate. Replace the plate with the included milling template, marked “3/8 EM 1.375 Deep From This Face.”

Step 6: Mill Receiver to 1.375”

Now, replace the original 3/8” drill bit with the short 3/8” end mill bit. For greater accuracy, we recommend installing the drill stop collar along the shaft of the bit and use the digital calipers to verify the depth at 1.375” precisely from the template plate. You’ll wish to let the shaft of the bit ride the interior shape of the plate to ensure an appropriate cut. Throughout this process, apply copious amounts of machining oil to the end mill bit and the template plate.

For the best cut, use shallow, precise movements to remove excess aluminum from the receiver. In doing so, you’ll avoid excess chatter.

If you are using a drill press for this step, which is designed to cut vertically solely, consider plunge-cutting, as the included end mill bits are center-cut and, thereby, designed for vertical and lateral cuts. You’ll use the end mill bit similar to a drill bit, plunging downward until you reach the 1.375” depth mark. With the pilot holes drilled, the aluminum left in the fire control cavity may then be removed using the same method. If you find burs or rough patches, polish and lightly mill them.

Step 7: Mill Receiver to 2.000” Deep

In step seven, we’ll return the mill receiver to a 2.000” depth. By now, you have the rear shelf, and most of the primary receiver cavity machined. You may now remove the short end bit and first milling template. Now, install the long 3/8” end mill bit and bolt the second milling template to the jig. This new plate includes a smaller interior guide and says “3/8” EM 2.00 Deep From This Face.”

Set the depth to stop precisely 2.000” deep from the face of the template using a stop collar and calipers.

Step 8: Cutting Out the Trigger Slot

With the central portion of the receiver cut to final depth ranges, both the 3/8” end mill bit and second milling template may be removed. Now, install the long 5/16” end mill bit and bolt the last template plate, marked “5/16” EM Thru Bottom,” to the jig. This step does not require precise measurements. You’ll cut through the floor of the 80% lower to create a slot for the trigger once done.

Once the trigger slot is cut, the receiver’s fire control cavity is finished. You may remove both the jig and the lower from your vise and position the entire assembly horizontally for further drilling. You want the right side faced.

Remember: Do not remove the template holder from the top of the jig. Leave it in place for now.

Step 9: Drill Hammer, Trigger, and Safety Pin Holes

The final step, now with the receiver and jig horizontally placed, is to drill the hammer, trigger, and safety pinholes. Drill entirely through the sides of the receiver via the right side plate. The drill bushings ensure every pin hole is aligned.

Using the 3/8” drill bit, complete the safety lever pinhole.

Using the 5/32” drill bit, finish the hammer pin hole, trigger, and disconnector pinhole.

You’re done! Your 80% lower is now ready for assembly. If you wish, you may coat the interior of the receiver to better protect the now-raw aluminum showing through. Some leave it bare, however.

Make it Easier with 80% Arms

Do these instructions look confusing, or you just aren't sure what they mean? No Problem. We've made things easier than ever with the release of our Easy Jig® Gen 3! Check out the product page for videos of how simple it really is, or our instruction manual page to see how simple we have made all of our products. Also, don't forget, we offer the Foolproof Guarantee! If you happen to mess up one of our lowers with one of our jigs, we'll replace it at a 50% discount.