The thought of completing your own 80% AR-15 lower may seem a little intimidating at the start – but don’t be worried. With a few simple tools, most anyone can accomplish the task of building their own AR-15.
To have a finished product that is professional looking and operates optimally, simply take your time and follow these instructions step by step.
Complete An 80% AR Lower Receiver
Before you begin your 80% AR lower receiver project, you will need to acquire some necessary tools. Remember, high quality end mills and drill bits will perform better and provide a much more professional looking lower.
Here is the list of tools necessary to complete your 80% AR lower receiver.
- 5/32” Drill Bit
- 3/8” Drill Bit
- 5/16” Drill Bit
- 3/8” End Mill
- Dremel with 1⁄4” Grinding Head
- Drill Press Vise
- Drill Press
AR-15lowerreceivers.com 80% Jig
Once you have the necessary tools, begin by assembling the 80% Jig. First, locate the side plates marked A & B. Place side plates A & B in the appropriate and corresponding holes for the lower receiver rear takedown pin.
Next, install the Top Plate marked VA-C. Insert bolts and assemble the jig “loosely,” then tighten all bolts once the jig is placed properly in the holes used for the rear takedown pin and sitting on the trigger well.
Top Plate C of the 80% jig, is clearly marked 1- 1⁄4” deep. This is the profile you should follow to make your cuts. You will need to drill 1- 1⁄4” deep from the time the bit touches the material. So, set the stop on the drill press accordingly when initially setting up your jig.
Start with a 3/8” drill bit and begin by drilling each radius corner in Top Plate C. This will make it easier when switching to the end mill later on. Below is a picture of the initial drill locations within Top Plate C.
Once each radius corner is drilled, move on to the second portion of Step 1. It is now time to remove as much material as possible with the drill. While drilling be sure to follow the pattern illustrated below and do not overlap the holes as you drill. Overlapping can cause unnecessary wear to the bit (or possible breakage) and make the drilling process much more difficult. (Please see picture and drilling pattern marked in red X’s below.)
It is now time to remove the aluminum “webs” created during the drilling process. Using a 3/8” end mill, plunge the end mill directly down onto the webs. This will remove the remaining aluminum and open up the fire control group cavity.
Continue the plunging motion until you have removed all the “webbing” and drill marks left on the bottom of the receiver from from the previous operation.
The overall depth of the end mill should also be 1-1/4-inches. This should be the finished depth of the fire control group cavity. By using the end mill you will also be creating a flat bottom within the 80% lower receiver.
After you remove all the aluminum webbing, it’s now time to remove Top Plate VA-C and change to Top Plate VA-D. You will also need to switch back to the 3/8” drill bit.
Repeat the previous steps (Step 1 and Step 2) with plate VA-D. Like plate VA-C, VA-D has a depth of 5/8” marked for you.
Once you have drilled the initial holes with the 3/8” bit, move on to Step 2 and use your 3/8” end mill to clear the remaining aluminum web material.
With the VA-D plate attached, switch to your 5/16” drill and drill completely through the fire control group cavity in the slot provided. Once drilled, clean it up with a hand file or 1⁄4” head in your Dremel. Be careful the drill does not ‘walk’ during this step of the process and again, do not overlap the holes when drilling.
Step 5 of the process is pretty straightforward. You will now drill into the marked side plates. With the 80% Lower and 80% Jig on their side you will see the two trigger-pin holes, marked 5/32” and your safety selector hole marked 3/8”.
Notice: We DO NOT suggest drilling all the way through the jig in a single step. Drilling all the way through may cause ‘walk’ if the wall on the other side of the fire control group cavity isn’t perfectly square.
To complete this step, drill only half way through the lower from one side, then flip the 80% jig and lower over and repeat.
Remove the jig and smooth the internal profile of the fire control group cavity with a Dremel tool or file. You can also reassemble the jig from time to time and check the quality of your work.
In order to finish the lower and/or allow more room to work, we recommend removing the lower from the jig. If preferred, you can leave the jig assembled for finishing everything but the trigger slot. Finishing that portion will require the lower be removed from the jig.
One these steps are completed you’ve earned the satisfaction of having legally finished your own 80% lower receiver under the Gun Control Act of 1968.
For further questions regarding the completing of your 80% AR lower receiver please contact AR-15 Lower Receivers for additional details.