Troubleshooting Common Issues When Finishing Your 80 Lower Receiver

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Building your own firearm from an 80 lower receiver is an exciting project that allows for customization and hands-on learning. However, even with careful planning and high-quality tools, it’s not uncommon to encounter some challenges along the way. Whether you’re drilling holes, milling the fire control cavity, or assembling the lower receiver, mistakes can happen, especially if you’re new to machining.

This guide will walk you through some of the most common issues builders face when completing an 80% lower receiver and how to troubleshoot and resolve them. With the right approach, you can overcome these challenges and complete your build successfully.

Common Issues When Drilling Holes

Problem 1: Misaligned Holes

Misaligned holes for the trigger pins, safety selector, or other components can prevent your firearm from functioning properly.

Possible Causes:

  • Incorrect setup of the 80% lower in the jig.
  • Drill bit wandering due to insufficient stability.
  • Failure to follow the jig’s guidelines or templates.

Solution:

  1. Double-Check Alignment: Before drilling, ensure the lower receiver is properly seated and secured in the jig. Use a vise to keep it stable.
  2. Use a Drill Press: A drill press offers greater precision compared to a hand drill.
  3. Pre-Punch Pilot Holes: Use a center punch to mark drill points. This helps prevent the drill bit from wandering.
  4. Take Your Time: Drill slowly and steadily to maintain control and accuracy.

Problem 2: Oversized or Over-Drilled Holes

Holes that are too large or drilled too deep can lead to loose-fitting pins and components.

Possible Causes:

  • Using the wrong drill bit size.
  • Over-drilling due to ignoring depth guides.

Solution:

  1. Verify Drill Bit Size: Double-check your jig kit instructions to confirm the correct bit sizes for each hole.
  2. Set a Drill Stop: If using a drill press, set a depth stop to prevent over-drilling.
  3. Repair Oversized Holes: If a hole is slightly oversized, you can use a roll pin or oversized pin to compensate. For severe issues, you may need to consult a gunsmith.

Common Issues When Milling the Fire Control Cavity

Problem 3: Uneven Milling

Uneven milling can result in a fire control cavity that doesn’t fit the trigger group properly, causing functional problems.

Possible Causes:

  • Inconsistent pressure while milling.
  • Incorrect alignment of the lower receiver in the jig.
  • Skipping steps or taking too large passes.

Solution:

  1. Use Small Passes: Remove material in thin layers to maintain control and accuracy.
  2. Check Progress Frequently: Stop periodically to measure the depth and ensure the cavity is uniform.
  3. Use a High-Quality Router or Milling Machine: A reliable tool will help you achieve consistent results.

Problem 4: Over-Milling

Removing too much material can compromise the structural integrity of your lower receiver, making it unsafe to use.

Possible Causes:

  • Failing to monitor progress against the jig’s depth guides.
  • Using the wrong end mill or router bit.

Solution:

  1. Follow Depth Guides Closely: Your jig kit will specify the correct depth for the fire control cavity. Measure regularly to avoid going too deep.
  2. Switch to a Precision End Mill: Ensure you’re using the correct bit for your specific jig and lower receiver.

Problem 5: Rough Edges or Burrs

Sharp edges or burrs in the fire control cavity can interfere with the movement of the trigger group and other components.

Solution:

  1. Deburr the Edges: Use a deburring tool or fine file to smooth out rough spots.
  2. Clean Thoroughly: Remove all metal shavings and debris using compressed air or a brush.

Assembly Problems

Problem 6: Components Don’t Fit Properly

After completing the machining process, you may find that components like the trigger, safety selector, or magazine catch don’t fit as expected.

Possible Causes:

  • Incorrectly drilled or milled areas.
  • Using incompatible parts.

Solution:

  1. Verify Parts Compatibility: Ensure you’re using components designed for your specific AR-15 platform and caliber.
  2. Inspect Machined Areas: Check for rough edges or debris that might be obstructing proper fitment. Smooth or clean these areas as needed.
  3. Adjust with Precision Tools: Use fine files or sandpaper to make minor adjustments if necessary.

Problem 7: Trigger Group Doesn’t Function Smoothly

A sticky or non-functional trigger group can render your firearm unreliable.

Possible Causes:

  • Misaligned or unevenly milled fire control cavity.
  • Incorrect assembly of the trigger group.

Solution:

  1. Inspect the Fire Control Cavity: Ensure the cavity is uniform and free of burrs or obstructions.
  2. Reassemble Carefully: Follow the lower parts kit instructions to assemble the trigger group correctly.
  3. Test Functionality: Perform a dry-fire test to check for smooth operation.

Legal Considerations

Problem 8: Serialization and Compliance Issues

Some states require serialization or registration of firearms built from 80% lowers. Failing to comply can result in legal issues.

Solution:

  1. Check Local Laws: Research your state and local regulations before completing your 80% lower.
  2. Apply for a Serial Number: If required, apply for a serial number through your state’s regulatory agency and engrave it on the lower receiver.
  3. Consult an Expert: If you’re unsure about the legal requirements, consult a firearms attorney or local FFL dealer.

Common Mistakes and How to Avoid Them

Mistake 1: Skipping Cutting Fluid

Cutting fluid reduces heat and friction during drilling and milling, preventing damage to your tools and lower receiver.

Solution: Always apply cutting fluid before starting and reapply as needed throughout the process.

Mistake 2: Ignoring Safety Precautions

Working with power tools and metal shavings can be hazardous if proper safety measures aren’t taken.

Solution:

  • Wear safety glasses and gloves at all times.
  • Keep your workspace clean and free of distractions.

Mistake 3: Rushing the Process

Rushing through drilling or milling increases the likelihood of errors.

Solution: Take your time and focus on accuracy rather than speed. Completing an 80% lower is a skill-building experience, so patience pays off.

Testing Your Completed Lower

Once you’ve resolved any machining issues, it’s time to test your completed lower receiver.

Function Checks:

  • Trigger Test: Ensure the trigger resets and releases the hammer properly.
  • Safety Selector Test: Confirm that the safety selector effectively blocks the trigger.
  • Magazine Fit Test: Insert and remove a magazine to ensure proper function.

Range Test:

After assembling your firearm, test it at a qualified shooting range to confirm reliability and performance.

Final Thoughts

Completing an 80% lower receiver comes with its challenges, but troubleshooting common issues is part of the learning process. By understanding potential problems and how to address them, you can build a safe, functional, and reliable firearm.

Ready to start your build? Check out Thunder Tactical for premium 80% lowers.