How to Drill and Mill an 80% Lower: Best Practices for Beginners

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Completing an 80 lower receiver for your AR-15 or AR-10 can be a rewarding project that gives you full control over your firearm build. However, if you’re new to machining, the process of drilling and milling might feel a bit intimidating. The good news? With the right tools, techniques, and preparation, even beginners can achieve professional-quality results.

This guide will walk you through the basics of drilling and milling an 80% lower receiver, along with best practices to ensure a smooth and successful project.

What Does Drilling and Milling an 80% Lower Involve?

An 80% lower receiver is unfinished and requires additional machining to function as the core of a firearm. Specifically, you’ll need to:

  1. Drill Pilot Holes: Create precise holes in the fire control cavity.
  2. Mill the Fire Control Cavity: Remove material to shape the cavity where the trigger and other fire control components will fit.
  3. Deburr and Clean: Smooth rough edges and remove debris for proper fit and finish.

Tools You’ll Need

Before you start, gather these essential tools:

1. 80% Lower Jig Kit

  • Guides your drill and milling machine to ensure precision.
  • Choose a jig compatible with your specific lower receiver.

2. Drill Press or Hand Drill

  • A drill press offers better stability and precision, while a hand drill is more portable.

3. Router or Milling Machine

  • Used to mill out the fire control cavity with accuracy.

4. Drill Bits and End Mills

  • Most jig kits include the required drill bits and end mills. Common sizes include 5/16″ and 3/8″.

5. Safety Gear

  • Safety glasses, gloves, and hearing protection are non-negotiable.

Step-by-Step Guide to Drilling and Milling an 80% Lower

Step 1: Set Up Your Workspace

Choose a clean, well-lit workspace with a sturdy surface. Ensure your tools are secure and within easy reach.

Pro Tips:

  • Use a vise to hold the jig and 80% lower securely.
  • Wear safety glasses and gloves to protect yourself from metal shavings.

Step 2: Secure the Lower Receiver in the Jig

Position the 80% lower in the jig according to the manufacturer’s instructions. The jig serves as a guide for drilling and milling, so double-check that everything is aligned correctly.

Pro Tips:

  • Tighten the jig securely to prevent movement during machining.
  • Check that the lower receiver is level to ensure even cuts.

Step 3: Drill Pilot Holes

Use your drill press or hand drill to create pilot holes in the fire control cavity. These holes make it easier to remove material when milling.

Best Practices:

  • Use Cutting Fluid: Reduces friction and heat, prolonging the life of your drill bits.
  • Drill Slowly and Steadily: Rushing can cause the drill bit to wander or overheat.
  • Follow Depth Guides: Your jig will indicate how deep each hole should be.

Step 4: Mill the Fire Control Cavity

After drilling the pilot holes, switch to a router or milling machine to remove the remaining material in the fire control cavity.

Best Practices:

  • Take Small Passes: Remove material in thin layers to maintain control and accuracy.
  • Check Your Progress Frequently: Compare your work to the jig’s guidelines to avoid over-milling.
  • Keep the Work Area Clean: Use compressed air or a brush to remove shavings as you go.

Step 5: Drill Selector and Pin Holes

Once the fire control cavity is complete, drill the holes for the safety selector and trigger pins. These are critical for the proper function of your completed firearm.

Best Practices:

  • Use the Jig’s Guides: Ensure precise placement of the holes.
  • Double-Check Bit Sizes: Verify you’re using the correct drill bits for these smaller holes.

Step 6: Deburr and Clean the Lower Receiver

After all drilling and milling are complete, inspect the lower receiver for burrs or rough edges.

How to Deburr:

  • Use a deburring tool or fine file to smooth sharp edges.
  • Clean the receiver thoroughly with compressed air or a brush to remove any remaining debris.

Common Mistakes to Avoid

  1. Skipping Cutting Fluid: Overheating your drill bits or end mills can damage your tools and the lower receiver. Always use cutting fluid.
  2. Rushing Through Milling: Removing too much material at once can lead to uneven cuts or damage. Take your time.
  3. Ignoring Depth Guides: Milling or drilling too deep can ruin the receiver and make it unsafe to use.
  4. Improper Alignment: Ensure the lower is properly seated in the jig to avoid misaligned holes or cuts.

Testing Your Finished Lower Receiver

Before assembling your firearm, perform a few simple tests:

  • Visual Inspection: Check for uneven milling or misaligned holes.
  • Parts Fit Test: Insert the fire control group components (trigger, hammer, safety selector) to ensure proper fit and function.
  • Safety Check: Confirm that the safety selector works as intended.

Staying Legal

Federal Guidelines

  • You can legally complete an 80% lower for personal use under federal law.
  • The finished lower must comply with all other firearm regulations (e.g., no fully automatic weapons).

State and Local Laws

Some states require serialization or prohibit the use of 80% lowers altogether. Always check your local regulations before starting your build.

Final Thoughts

Drilling and milling an 80% lower receiver is a rewarding way to build a custom firearm while learning about its mechanics. By following best practices and using the right tools, even beginners can complete a lower receiver with precision and confidence.

Ready to get started? Check out Thunder Tactical for high-quality 80% lowers, jig kits, and all the tools you need to complete your AR-15 build. Happy building!