Understanding the Fundamentals
The Caliber Breakdown
Before diving into the specifics of a 9mm upper on a 5.56 lower, it’s crucial to understand the core differences between the two calibers. The 5.56 NATO round, the standard for many military and law enforcement agencies, is a high-velocity, intermediate cartridge designed for long-range accuracy and penetration. It boasts a significant effective range and excellent terminal ballistics. In contrast, the 9mm Parabellum (9x19mm) is a shorter, lower-velocity pistol cartridge. Historically, the 9mm has been the dominant caliber for handguns. The 9mm offers lighter recoil, making it easier to control, and its ammunition is typically more affordable than 5.56. The 9mm excels in close-quarters combat and is a favorite among competition shooters.
The AR-15 Platform
The AR-15’s modularity is the foundation for the 9mm upper on a 5.56 lower concept. The AR-15 platform is built around two main components: the upper receiver and the lower receiver. The upper receiver houses the barrel, bolt carrier group (BCG), and handguard. The lower receiver contains the fire control group (trigger, hammer, etc.), the buffer system, and the magazine well. This split design allows for a simple swap of the upper receiver to change calibers. The 9mm conversion leverages this very feature.
The 9mm Upper Components
For the 9mm upper, the key is the bolt carrier group which is designed to work with 9mm cartridges. The 9mm BCG has a different bolt face and firing pin to accommodate the 9mm case rim and primer. The barrel is also designed specifically for 9mm rounds, with a rifling twist rate optimized for these projectiles. The lower receiver, although technically compatible, requires further consideration as the magazines are different.
Compatibility: Making it Work
Lower Receiver Compatibility
The beauty of using a 9mm upper on a 5.56 lower is that, in many instances, the lower receiver itself is compatible. However, the lower receiver’s features may require specific attention.
A standard 5.56 lower receiver is made to accept 5.56 magazines. Since 9mm rounds are smaller and loaded into a magazine with a different geometry, you will need a 9mm-specific magazine. The most common solution is to use a 9mm magazine adapter or to modify the lower receiver to accept 9mm-specific magazines. There are two basic options for this: Colt-pattern magazines (which fit in a special adapter) and Glock-pattern magazines. The latter is a more common choice.
Magazine Considerations
When considering magazine compatibility, it’s essential to identify the method of feeding. Some manufacturers offer lowers specifically built for 9mm magazines that can use Glock-style magazines. You can also often find magazine adapters that utilize the same lower receiver as the 5.56 rifle and allow it to receive 9mm magazines. These adapters typically fit into the magazine well. Some manufacturers offer products to achieve this conversion.
Building or Buying Your Conversion
Build vs. Buy
You have a couple of paths to achieve the 9mm upper on a 5.56 lower setup: building your own or purchasing a pre-built upper. Both approaches have their benefits.
Building your own upper gives you the most control over component selection and allows for greater customization. It enables you to tailor the barrel length, handguard style, and other features to your specific preferences. Building also enables the owner to gain more familiarity with the operating parts, thereby helping during maintenance. However, building requires some mechanical aptitude, the right tools, and a good understanding of how an AR-15 functions.
Building Your Own 9mm Upper
If you decide to build a 9mm upper, here’s what you need:
- 9mm Barrel: The heart of the system. Choose a barrel length that suits your intended purpose (e.g., pistol-length for compact builds, longer for increased accuracy).
- 9mm Bolt Carrier Group (BCG): Ensure compatibility with the 9mm cartridge. This is specifically designed to interface with the 9mm barrel and provide the necessary action to feed the round.
- Upper Receiver: Standard AR-15 upper receivers are compatible.
- Handguard: Choose a handguard that complements your barrel length and desired accessories. Consider M-LOK or KeyMod for ease of accessory mounting.
- Charging Handle: Select a charging handle that suits your preference. Extended handles are great for easy operation.
- Muzzle Device: Flash hiders or compensators are often used to manage recoil and muzzle climb.
Purchasing a Pre-Built Upper
Purchasing a pre-built upper is a convenient option, particularly for those new to the AR-15 platform or those who prefer a hassle-free approach. Reputable manufacturers offer high-quality 9mm uppers with pre-assembled components, simplifying the process. This option saves time and often provides a warranty on the components.
When purchasing a pre-built 9mm upper, research the manufacturer thoroughly, and read reviews from other owners. Pay attention to the included features, such as feed ramps (crucial for smooth feeding) and ramped bolts (to further aid reliability).
Essential Adaptations and Upgrades
Magazine Adaptations
Beyond simply swapping the upper receiver, specific modifications can enhance your 9mm upper on a 5.56 lower setup.
A key consideration is magazine compatibility. As mentioned, the 5.56 lower receiver’s magazine well isn’t designed for 9mm magazines. Therefore, you’ll need a method to use 9mm magazines. This is achieved through magazine adapters or dedicated 9mm-specific lowers. Adapters drop into the standard magazine well, allowing for 9mm magazines to lock into place. Another key method to convert is purchasing a 9mm specific lower receiver or lower-parts kit to achieve a 9mm configuration.
Buffer System Modifications
The buffer and buffer spring also need attention. 5.56 lowers typically utilize a buffer and spring designed for that caliber. A 9mm setup often benefits from a heavier buffer weight and a specific buffer spring. This is because the 9mm round produces less gas than the 5.56. If the buffer is too light, the bolt carrier group may cycle too quickly, leading to malfunctions. A heavier buffer slows the cycling and increases the reliability. Many aftermarket manufacturers supply buffer systems designed specifically for the 9mm, and this will provide a more reliable and enjoyable shooting experience.
Advantages of the Conversion
Cost Savings
The 9mm upper on a 5.56 lower configuration boasts several advantages:
- Cost-Effectiveness: 9mm ammunition is typically cheaper than 5.56 NATO, especially during periods of high demand or supply chain disruptions. This can make training and recreational shooting more affordable.
Recoil Reduction
- Reduced Recoil: The 9mm cartridge produces significantly less recoil than the 5.56. This facilitates faster follow-up shots, enhances control, and makes the rifle easier to handle, especially for new shooters or those with physical limitations.
Enhanced Control
- Increased Controllability: The lighter recoil makes the 9mm AR-15 easier to control, allowing for faster target acquisition and more accurate shot placement.
Quieter Operation
- Quieter Operation (with Suppressor): When paired with a suppressor, the 9mm offers a quieter shooting experience compared to suppressed 5.56. The reduced noise signature can be beneficial for certain shooting environments and situations.
Potential Downsides and Troubleshooting
Reliability Considerations
Despite the benefits, there are potential downsides to using a 9mm upper on a 5.56 lower.
Reliability issues can arise due to improper component selection or assembly. Feeding problems can occur if the feed ramps aren’t properly designed or if the magazines aren’t compatible. Extraction issues are another area to be aware of, as the bolt might not extract the spent casing reliably.
Troubleshooting Common Issues
Troubleshooting often involves careful examination of the components. Ensure the barrel and bolt carrier group are compatible, that the magazine is the correct type, and that the buffer system is appropriately weighted. If you experience failure to feed, check the magazine and feed ramps. For extraction issues, inspect the extractor and ejector.
Maintenance and Keeping Your System Running
Maintaining your 9mm upper is crucial for optimal performance and longevity. After each shooting session, thoroughly clean the barrel, bolt carrier group, and the receiver. Remove any carbon buildup or debris. Lubricate the BCG properly, using a high-quality gun lubricant. Regular maintenance ensures your firearm operates smoothly and reliably.
Safety First
Always prioritize safety when handling any firearm, including a 9mm upper on a 5.56 lower. Always follow these safety rules:
- Treat every firearm as if it is loaded.
- Never point the muzzle at anything you don’t intend to shoot.
- Keep your finger off the trigger until you are ready to fire.
- Be aware of your target and what is beyond it.
Proper storage is also critical. Store the rifle unloaded, separate from ammunition, and in a secure location, away from unauthorized users.
Conclusion
Converting your 5.56 AR-15 to 9mm with a 9mm upper offers a compelling option for shooters seeking a versatile, cost-effective, and controllable firearm. While there are considerations for compatibility and component selection, the conversion is generally straightforward. By understanding the fundamentals, choosing the correct components, and adhering to proper maintenance and safety practices, you can enjoy the benefits of this popular conversion. The choice to take on the 9mm upper on a 5.56 lower is ultimately a personal one that should be based on your budget, shooting goals, and commitment to the platform.
