Building a 9mm pistol caliber carbine (PCC) using a 9mm upper receiver on a standard AR-15 5.56 lower receiver is a popular choice among firearm enthusiasts. This configuration offers a cost-effective way to enjoy the versatility and familiarity of the AR-15 platform while shooting the affordable and readily available 9mm ammunition. However, there are important considerations and potential challenges to understand before embarking on this build. This in-depth guide explores the intricacies of using a 9mm upper on a 5.56 lower, covering everything from compatibility issues to performance expectations.
Understanding the Conversion: 9mm Upper on 5.56 Lower
The core concept behind this conversion lies in utilizing the existing AR-15 lower receiver, the firearm's "brains," with a new upper receiver designed specifically for 9mm ammunition. The lower receiver houses the fire control group (trigger, hammer, etc.), which remains largely unchanged. The 9mm upper, however, necessitates a different bolt carrier group (BCG), barrel, and magazine well to accommodate the 9mm cartridge. This modularity is a key advantage of the AR-15 platform.
Key Components in a 9mm AR-15 Conversion
- 5.56 Lower Receiver: The foundation of the build, this component remains unchanged. Ensure it's in good working order.
- 9mm Upper Receiver: This houses the 9mm specific components. Choose a reputable brand known for quality and reliability.
- 9mm Bolt Carrier Group (BCG): This is crucial. It's designed specifically for 9mm operation and cannot be used with 5.56 ammunition.
- 9mm Barrel: This component dictates the accuracy and performance of the firearm. Barrel length choices affect velocity and overall performance.
- 9mm Magazine Well: Adapts the lower receiver to accept 9mm magazines, usually Glock or Colt style.
- 9mm Magazines: Select high-quality magazines from reputable manufacturers for reliable feeding.
Compatibility and Considerations: Choosing the Right Parts
While the basic concept seems simple, achieving reliable function requires careful component selection. Incompatibility can lead to malfunctions like failure to feed, failure to extract, or even dangerous malfunctions.
Choosing a Compatible Upper Receiver
The 9mm upper receiver must be designed to work flawlessly with a standard AR-15 lower receiver. Look for uppers that are specifically stated as compatible with your chosen lower receiver and type of 9mm magazine (Glock, Colt, etc.). Cheap or poorly made uppers are far more likely to have issues.
Bolt Carrier Group (BCG): A Critical Component
The BCG is the heart of the 9mm upper. It's essential to select a BCG from a reputable manufacturer. Using a poorly made or incompatible BCG is a recipe for disaster. Never use a 5.56 BCG in a 9mm upper, and never use a 9mm BCG in a 5.56 upper. Doing so can cause catastrophic damage to the firearm and serious injury.
Magazine Compatibility: Glock, Colt, or Other?
Many 9mm AR-15 uppers utilize Glock or Colt pattern magazines. Choose a system and stick with it. Mixing and matching magazines can lead to reliability problems. High-quality magazines are essential for reliable feeding.
Performance and Reliability: What to Expect
A properly built 9mm AR-15 conversion can be incredibly reliable and accurate. However, factors like component quality and ammunition selection heavily influence performance.
Accuracy and Precision
Expect accuracy comparable to other 9mm firearms. While not as inherently accurate as a precision rifle chambered in 5.56, a well-built PCC can easily hit targets at practical self-defense and recreational shooting ranges.
Reliability: Avoiding Malfunctions
A high-quality upper and BCG are key to reliable function. Ensure the upper and lower are properly mated and components are properly installed. Using high-quality ammunition also contributes to smooth operation.
Ammunition Considerations
Use high-quality 9mm ammunition designed for use in semi-automatic firearms. Avoid ammunition that is known to be unreliable.
Cost Comparison: Is it Worth it?
Building a 9mm AR-15 can be more cost-effective than purchasing a dedicated 9mm PCC from a manufacturer, especially if you already own an AR-15 lower receiver. However, the total cost of a conversion still depends on the quality of parts you choose.
| Component | Cost Range (USD) |
|---|---|
| 9mm Upper Receiver | $200 - $600 |
| 9mm Bolt Carrier Group | $100 - $300 |
| 9mm Barrel | $100 - $300 |
| Magazine Well | $50 - $150 |
| Magazines (Multiple) | $100 - $300+ |
Note: These are estimates and can vary based on brand and retailer.
Conclusion: A Viable and Versatile Option
Converting an AR-15 to 9mm by using a 9mm upper on a 5.56 lower offers a viable and versatile option for shooters seeking the advantages of the AR-15 platform with the affordability and versatility of 9mm ammunition. However, proper research, careful component selection, and meticulous assembly are crucial for ensuring reliable and safe operation. Remember to consult with experienced gunsmiths if you have any doubts or need assistance with the build process. Safety should always be the top priority.