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We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, New Jersey, New York, Pennsylvania, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.

How to Complete an 80% Lower Receiver

Posted by 80-Lower.com on Jul 28th 2020

How to Complete an 80% Lower Receiver

For this guide we're completing a forged AR-15 80% lower. These steps also apply to completing a billet and AR9 lower. We're also using the popular Elite Builder Jig, which comes with end mill bits and drill bits. These instructions will cover how to complete your 80% lower using either a drill press or milling machine.

New here? Not sure where to start? Check out these guides:


Tools Required

You'll need the following tools:

  • Drill press or milling machine
  • Vise for clamping the 80% jig
  • Machining oil for lubrication

Optionally, you can also use:

  • Digital calipers
  • Drill stop collars

The Elite Builder Jig includes:

  • (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”

If you're new to machining 80% lowers and don't know which tools are required or recommended and why, check out this guide: Tools required for finishing a lower. It covers using either a drill press or milling machine in greater detail, and we'll again cover both methods here.


Step 1: Secure Lower in Jig

Rest the 80% lower on the right jig plate so the pins on the plate rest inside the rear take-down and from pivot pin holes. Secure the left jig plate to the opposite side and stand the assembly up vertically.

Step 2: Install Template Plate Holder

Next, install the template plate holder. Seat the holder atop the jig so the rounded edge rests against the buffer tube housing. Bolt the holder to the side plates using the two largest hex-head bolts provided.

Step 3: Install Pilot Hole Template and Vise

Using the two small countersunk hex-head bolts, install the pilot hole drilling template plate into the template plate holder. This first plate is marked "3/8 DR 2.00 DEEP FROM THIS FACE". Then tighten the jig and lower into your vise for machining.

WARNING: Do not over-tighten the vise. Only light pressure is required to keep the jig from moving while you work. Over-tightening the vise will bend the side plate. This may cause misalignment of the jig and lower.

Step 4: Install 3/8" Drill Bit

Collect the 3/8" drill bit provided and install it into your drill press or milling machine. Align the jig and lower with your tooling, and begin drilling through each pilot hole guide on the template plate into the 80% lower receiver. You must drill exactly 2.000" deep from the top face of the template plate. Set your drill or mill's depth stop accordingly. 

Step 5: Drill Receiver Pilot Holes to 2.000" Deep

Lubricate the template plate's guides and the receiver with machining oil to enhance bit life with this step. You'll need this bit for completing the safety lever pin hole. Drill exactly 2.000" deep into the receiver from the face of the template plate. Set the depth stop on your drill or mill as needed, and optionally, use a stop collar and calipers to verify the final depth is 2.000". Once all pilot holes have been drilled, remove the 3/8" bit and pilot hole plate. Replace the plate with the first milling template. It is marked "3/8 EM 1.375" DEEP FROM THIS FACE".

Step 6: Mill Receiver to 1.375" Deep

IF USING A DRILL PRESS, READ THE "DRILL PRESS" SECTION FIRST.

Replace the 3/8" drill bit with the short 3/8" end mill bit. Optionally, install the drill stop collar on the shaft of the bit and use your calipers to verify the cutting depth is precisely 1.375" from the face of the template plate. Allow the shaft of the bit to ride along the interior shape of the plate. This ensures the shape of the receiver is appropriately cut to size. Apply plenty of machining oil or lubricant to both the end mill bit, the work area, and the template plate. With this template, It is recommended you remove aluminum in shallow incremental passes as you reach the final cutting depth to reduce bit run-out and chatter. 

Using a Drill Press to Cut the Receiver

Unlike a milling machine, which can cut almost any material laterally (side to side), a drill press is designed to provide only vertical machining. Milling machines use quills, spindles and chucks that are threaded and locked to prevent the tooling from falling out of the machine when placed under lateral loads. However, many drill presses use tapered spindles and chucks, which are press-fit onto the machine. When high amounts of lateral load are applied, the press-fit spindle and chuck is at higher risk of falling from the machine. To safely cut the 80% lower's fire control cavity using the end mill bits with a drill press, two methods can be used:

Plunge-cutting

The provided end mill bits are center-cut. That means they are capable of cutting both vertically and laterally. Cutting vertically is easy: Simply use the end mill bit like a drill bit, plunging downward to the final depth of 1.375". With the pilot holes drilled, the remaining portion of aluminum in the fire control cavity can be removed in this fashion. Any burs and rough spots can be polished and lightly milled. Continue reading before machining:

Light milling with safety precautions

Aluminum -- whether forged or billet in construction -- is a soft metal that can be easily cut without high load or pressure required. As such, some 80% builders choose to simply employ their drill press as a milling machine for these steps. If you choose to mill the receiver laterally with your drill press, you must employ appropriate PPE: Eye protection, work gloves, and hearing protection should be worn. Cutting passes should be made shallow, removing only a small layer of aluminum at a time. Inspect the quill, chuck, and spindle to ensure the tooling is tight and free of play. If your machine experiences excess vibration or tooling chatter, do not continue cutting laterally. Only employ plunge cuts to vertically cut the receiver.

Step 7: Mill Receiver to 2.000" Deep

With the rear shelf and part of the main cavity machined, the short bit and first milling template can be removed. Install the long 3/8" end mill bit in your machine and bolt the second milling template to the jig. This plate has a smaller interior guide and is marked "3/8 EM 2.00 DEEP FROM THIS FACE". Whether using a mill or drill press, repeat the same machining steps as step 6: Set your depth stop to 2.000" deep from the face of the template, optionally using a stop collar and calipers to verify your measurement.

Step 8: Cut the Trigger Slot

With the main cavity cut to its final depth, the 3/8" end mill bit and second milling template can be removed. Install the long 5/16" end mill bit into your machine and bolt the final cutting template marked "5/16" EM THRU BOTTOM" to the jig. This doesn't require any precise measuring; simply cut through the floor of the receiver to make the slot for the trigger. Once this final cut has been made, the receiver's fire control cavity will be complete. You can remove the jig and lower from the vise to reposition the assembly horizontally for drilling. Ensure the right side of the jig is faced.

WARNING: Do not remove the template holder atop the jig. 

The template holder provides a critical connection between both side plates on the jig. Removing the template holder will allow each side plate to move independent of the other, resulting in misaligned holes drilled for the parts kit's pins. Leave the template holder bolted to the jig, and reposition the lower and jig in the vise horizontally. Do not over-tighten the vise. Over-tightening the vise will cause the side plates to warp, resulting in misaligned pin holes.

Step 9: Drill Hammer, Trigger, Safety Pin Holes

Drill completely through both sides of the receiver via the jig's right side plate only. The drill bushings will ensure each pin hole remains aligned as it is completed. Use the 3/8" drill bit to complete the safety lever pin hole. Use the 5/32" drill bit to complete the hammer pin hole and trigger and disconnector pin hole.


Your 80% Lower is Ready for Assembly!

With the pin holes drilled, your 80% lower is complete! It is now a proper AR-15 stripped lower receiver that's ready for a parts kit install. You may choose to coat the inside of the receiver to protect the raw aluminum, or leave it bare as a testament to your handiwork. 

Next up, the final assembly: Check out the instructions for How to Install the AR-15 Lower Parts Kit.

Have questions? Get in touch with us here. (A real person will answer the phone!)

DISCLAIMER: If you are new to the world of DIY gun 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. At 80-lower.com, 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.

We are a national retailer of individual components and not all products depicted on this website are legal in every state. Shipping of various products found on this website are prohibited to some states (such as California, Connecticut, District of Columbia, New Jersey, New York, Rhode Island, and Washington). The information, pictures, text or products presented on this website are not a representation by us, and should not be understood by you, that any product or completed firearm is legal to assemble or own in your state of residence. We encourage each and every builder to perform their own research about the state and federal laws that apply to them. It is your responsibility to understand the law and we encourage you to consult with an attorney or your local ATF representative.