How to Safely and Legally Finish Your 80% Lower Receiver

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Completing an 80% lower receiver is a rewarding project that allows you to build your own AR-15, AR-10, or other firearms. However, to ensure your build is safe, functional, and compliant with all legal requirements, it’s essential to follow the correct steps and understand the laws surrounding 80 lowers.

In this guide, we’ll walk you through the process of safely and legally finishing your 80% lower receiver, covering tools, techniques, and key regulations to keep in mind.

Step 1: Understand the Legal Basics

Federal Laws on 80% Lowers

Under federal law, 80% lower receivers are not considered firearms because they are unfinished and cannot function as such. You can legally purchase and complete an 80% lower for personal use without requiring:

  • A background check.
  • An FFL transfer.
  • A serial number (in most states).

However, the finished receiver becomes a firearm and is subject to all laws governing firearms. You cannot:

  • Manufacture firearms for sale or transfer without a license.
  • Build a fully automatic weapon.

State-Specific Laws

Some states impose additional requirements, such as serialization or registration of firearms built from 80% lowers. Check your state and local laws to ensure compliance.

Step 2: Gather the Right Tools and Materials

Completing an 80% lower requires specific tools to ensure precision and safety. Here’s what you’ll need:

Essential Tools

  1. 80% Lower Jig Kit: Guides your drilling and milling to ensure accuracy.
  2. Drill Press or Hand Drill: For drilling holes in the lower receiver.
  3. Router or Milling Machine: To remove material from the fire control cavity.
  4. Vise: Holds your 80% lower securely in place while you work.
  5. Safety Equipment: Always wear safety glasses, gloves, and hearing protection.

Additional Materials

  • Drill Bits and End Mills: Typically included with your jig kit.
  • Cutting Fluid: Reduces heat and friction during machining.
  • Deburring Tools: Smooths rough edges after machining.

Step 3: Set Up Your Workspace

Create a safe and organized workspace to complete your 80% lower receiver.

Tips for a Safe Workspace

  • Well-Lit Area: Ensure good visibility to avoid mistakes.
  • Stable Workbench: Use a sturdy bench to secure your tools and materials.
  • Ventilation: Machining generates metal shavings and dust, so work in a well-ventilated area.

Step 4: Secure the Lower in the Jig

Attach your 80% lower receiver to the jig kit according to the manufacturer’s instructions. The jig serves as a blueprint, guiding your drill and router to remove the correct material.

  • Double-Check Alignment: Ensure the lower is seated correctly to avoid uneven cuts.
  • Tighten Securely: Use a vise to keep the jig and lower from shifting during machining.

Step 5: Drill the Pilot Holes

Use the drill press or hand drill to create pilot holes in the fire control cavity. Follow the jig’s template for proper placement.

Drilling Tips

  • Use Cutting Fluid: Reduces friction and prolongs the life of your drill bits.
  • Go Slow: Drill at a steady pace to maintain control and prevent overheating.
  • Check Depth: Ensure you’re drilling to the correct depth as indicated by the jig.

Step 6: Mill the Fire Control Cavity

Switch to a router or milling machine to remove material from the fire control cavity. This step requires patience and precision.

Milling Tips

  • Take Small Passes: Remove material in small increments to maintain accuracy.
  • Check Progress Frequently: Compare your work to the jig template to avoid over-milling.
  • Clean as You Go: Remove metal shavings regularly to keep your workspace clear.

Step 7: Deburr and Clean the Receiver

Once machining is complete, inspect the fire control cavity and other drilled areas for rough edges or burrs.

  • Use a Deburring Tool: Smooth out sharp edges to ensure proper fitment of components.
  • Clean Thoroughly: Use compressed air or a brush to remove metal shavings and dust.

Step 8: Assemble the Lower Receiver

With your 80% lower receiver completed, it’s time to assemble the fire control group and other components using a lower parts kit (LPK).

Assembly Steps

  1. Install the Trigger and Hammer: Fit these components into the fire control cavity and secure them with the provided pins.
  2. Attach the Safety Selector: Install the selector and grip for proper functionality.
  3. Add the Magazine Catch and Bolt Catch: Secure these components in their designated slots.
  4. Install the Buffer Tube: Thread the buffer tube into the receiver and add the buffer spring and buffer.

Step 9: Perform a Function Check

Before attaching the upper receiver, test the functionality of your completed lower receiver.

  • Safety Selector Test: Ensure the safety blocks the trigger when engaged.
  • Trigger Reset Test: Pull the trigger, cycle the hammer, and listen for a reset click.
  • Magazine Fit Test: Insert a magazine to ensure it locks and releases properly.

Step 10: Stay Compliant

Once your 80% lower receiver is completed and assembled, it becomes a firearm subject to all applicable laws.

Key Compliance Tips

  • Serialization: If your state requires serialization, engrave a serial number on your lower receiver before assembling the firearm.
  • Feature Restrictions: Ensure your build complies with state-specific laws, such as bans on certain configurations or features.
  • No Sale or Transfer: Do not sell or transfer the firearm unless it is properly serialized and transferred through an FFL.

Safety Tips for Using Your Completed Firearm

  • Inspect Your Work: Ensure all components are securely installed and functioning properly before firing.
  • Use Quality Ammo: Always use ammunition that matches your firearm’s caliber.
  • Range Test: Test your firearm at a qualified shooting range to ensure reliability and performance.

Final Thoughts

Completing an 80% lower receiver is a fun and rewarding way to build your own firearm, but it’s essential to approach the process with care and attention to detail. By following these steps and staying informed about local and federal regulations, you can safely and legally create a custom firearm that fits your needs.

Looking for high-quality 80% lowers and jig kits? Check out Thunder Tactical for everything you need to start your next project with confidence. Happy building!