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A Complete Guide to Completing an 80% Lower

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If you are ready to complete your own 80% AR-15 lower receiver, you’ve reached a very exciting time in your life. Although the thought may be a bit overwhelming, getting the job done is not as difficult as you might think. As long as you have the right tools and a bit of patience, you can accomplish the task just fine. So let the games begin!

Step 1: Gather Your Tools

Your first step is to gather your tools. When selecting your a-co, the higher the quality, the better the performance. Choose tools that are suitable for your desired result.

Here’s what you will need to get started:

  • 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

Step 2: Begin the 80% Jig

Now it’s time to get started on the 80% Jig. Locate the side plates marked A & B. Place those in their corresponding holes for the lower receiver rear takedown pin.

Step 3: Install the Top Plate

The Top Plate is marked VA-C. Insert the bolts and assemble the jig loosely. Next, tighten all the bolts once the jig is properly placed in the holes that are used for the rear takedown pin and are resting on the trigger well.

Step 4: Make Your Cuts

Top Plate C of the 0% jig is marked 1-1/4” deep. Follow this profile to make your cuts. Drill 1-1/4” deep from the time the bit touches the material. Be sure to set the stop on the drill accordingly so you do not go deeper than 1-1/4”.

When you begin this step, start with a 3/8” drill and drill each radius corner in Top Plate C. Doing so will make it much easier when switching to the end mill later in the process. See the picture below of the initial drill locations within Top Plate C.

Step 5: Follow the Pattern

Once each radius corner is drilled, remove as much material as you can with the drill. As you are drilling, follow the pattern illustrated in the picture provided, and do not overlap the holes as you drill, as that can cause unnecessary wear and breakage.

Step 6: Remove the Aluminum Webs

Use a 3/8” end mill to plunge the end mill directly down onto the webs to remove the remaining aluminum and open the fire control group cavity. The end mill allows you to create a flat bottom within the 80% lower receiver.

Continue the plunging motion until all of the webbing and drill marks on the bottom of the receiver are removed.

The overall depth of the end mill should be 1-1/4”, which should be the final depth of the fire control group cavity.

Step 7: Change Top Plate VA-C to Top Plate VA-D

After all the aluminum webbing is removed, switch back to the 3/8” drill bit to change the Top Plates. Repeat steps 2-6 with Plate VA-D, which has a depth of 5/8”, marked.

Step 8: Drill the Fire Control Group Cavity

With the VA-D Plate attached, use the 5/16” drill to drill completely through the fire control group cavity in the slot provided. After you’ve drilled the hole, clean it up with a hand file or 1/4” head in your Dremel.

Do no let the drill walk and do not overlap the holes while you are drilling.

Step 9: Drill the Side Plates

There are two trigger-pin holes on the sides of the 80% lower and the 80% jig. The holes are marked 5/32” and your safety selector hole is marked 3/8”.  Drill halfway through the lower from one side, then flip the 80% jig and repeat.

We do not suggest drilling all the way through the jig in one single step because that may cause walk if the wall on the other side of the fire control group cavity isn’t perfectly square.

Step 10: Remove the Jig

The final step is to remove the jog and smooth the internal profile of the fire control group cavity with a Demel tool or file.

We recommend removing the lower from the jig to finish the lower to finish the trigger slot.

Once you’ve completed these 10 steps you are well on your way. If you have questions about the process or get stuck at a step, feel free to contact our team for additional details.


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