The .40 S&W Cartridge: A Closer Look
History and Development
Before venturing into the complexities of adapting an AR-15 for .40 S&W, it’s essential to understand the cartridge itself. The .40 S&W was born from a confluence of events in the late 1980s. Law enforcement agencies, looking for an intermediate round between the 9mm Luger and the .45 ACP, sought a cartridge that delivered a balance of stopping power and manageable recoil. The FBI’s investigation into the Miami shootout, where a higher round capacity was needed, highlighted this need and helped the birth of this cartridge. This led to a collaboration between Smith & Wesson and Winchester, resulting in the creation of the .40 S&W.
Ballistics and Performance
The .40 S&W was designed to replicate the performance of the 10mm Auto cartridge within a smaller, more manageable package that would fit in existing 9mm pistol frames. The .40 S&W typically fires bullets ranging from 155 to 200 grains. The ballistic characteristics place it firmly in the mid-range of handgun cartridges. A 180-grain bullet typically leaves the barrel at around 950 to 1,000 feet per second, generating significant energy for self-defense scenarios.
Applications
The cartridge quickly gained traction in law enforcement circles, becoming a favored choice for its perceived stopping power and capacity, often surpassing the 9mm in terms of energy and perceived effectiveness. Its popularity spread to the civilian market as well, offering a compelling alternative to the established standards. It found its way into a variety of handguns, including models from Glock, Smith & Wesson, SIG Sauer, and many others.
Advantages and Disadvantages
Despite its initial success, the .40 S&W has faced evolving opinions. While it has historically been lauded for its stopping power, some modern cartridges like the 9mm have seen advancements in bullet technology to create improved stopping power. The .40 S&W cartridge has also seen a more aggressive recoil profile than 9mm, requiring a more experienced shooter to handle the cartridge effectively. This means the cartridge’s popularity has seen a dip with some shooters. Nonetheless, it remains a viable choice for personal defense and target shooting, appreciated for its availability and its consistent ability to pack a punch.
Can You Do It: The Feasibility
Challenges
The idea of an AR-15 in .40 S&W presents a fascinating engineering challenge. The AR-15 platform is fundamentally designed around the dimensions and operating principles of rifle cartridges like the .223 Remington or 5.56x45mm NATO. Adapting it to a pistol cartridge like the .40 S&W requires careful consideration of several key components. One of the most significant hurdles is magazine compatibility. The AR-15’s lower receiver, designed to house standard rifle magazines, is not naturally suited for the shorter, wider .40 S&W cartridges. This necessitates the use of specialized magazines designed specifically for pistol-caliber carbines (PCCs). These magazines are typically designed to fit into the AR-15’s magazine well using an adapter or a dedicated lower receiver. Common magazine options include those compatible with Glock magazines, which are widely available and known for their reliability. Other options may involve using magazines designed for the Colt-pattern. The magazine choice will often dictate the lower receiver design as well, making the magazine selection the first consideration for building this type of firearm. Furthermore, the bolt and barrel assembly must be designed to handle the .40 S&W’s ballistics. The bolt must be appropriately sized to extract, eject, and chamber the shorter cartridge. The barrel requires a specific bore and chambering for .40 S&W, which differs from the standard .223/5.56 barrels. The rifling twist rate will also be different, optimized to stabilize .40 S&W bullets. The barrel needs to be properly headspaced to ensure safe and reliable operation. Therefore, finding a reliable bolt carrier group and a high-quality barrel from reputable manufacturers is essential. The buffer system plays a crucial role in managing the .40 S&W’s recoil. The AR-15’s buffer system works in tandem with the bolt carrier to absorb the energy generated during the firing cycle. The shorter, more powerful .40 S&W cartridge will require a buffer spring and a buffer weight that are designed to accommodate the recoil impulse. These components need to be tuned properly to ensure the firearm cycles reliably without excessive recoil or malfunctions. Too light of a buffer, or too heavy of a buffer, can create problems with the firearm’s action. Finally, the lower receiver itself might require some modifications, depending on the specific build and the magazine system selected. Some manufacturers offer dedicated lower receivers designed to accept specific magazine types (e.g., Glock magazines) and/or have modifications to function with the .40 S&W cartridge. This can often make building the AR-15 in .40 S&W a more streamlined process, offering a more reliable platform. In these cases, you may need to change the magazine release button or the magazine well shape to allow for proper magazine seating.
Availability of Components
The availability of components is an important consideration. While the AR-15 in .40 S&W is not as common as its 5.56 NATO counterparts, the necessary parts, including barrels, bolts, and magazines, are generally available from specialized manufacturers. Researching and choosing these components from reputable vendors is critical for building a safe and reliable firearm. This will involve some searching around and potentially more cost to obtain the specific components needed for the build. Despite these challenges, building an AR-15 in .40 S&W is certainly feasible, representing a specialized niche within the AR-15 world. It is a project that calls for careful planning, component selection, and a thorough understanding of the AR-15 platform and the .40 S&W cartridge.
Putting it Together: The Assembly Process
Component Selection
Building an AR-15 in .40 S&W is largely similar to assembling a standard AR-15, but with some key differences related to the caliber-specific components. Here’s a high-level overview. The selection of components is a critical first step. This includes carefully choosing:
- Lower Receiver: Select a lower receiver designed for the AR-15 platform, often optimized to accept pistol caliber magazines.
- Upper Receiver: A standard upper receiver is acceptable, although some manufacturers produce dedicated .40 S&W upper receivers with appropriately sized ejection port dimensions.
- Barrel: A barrel specifically chambered for .40 S&W with a suitable rifling twist rate is essential.
- Bolt Carrier Group (.40 S&W Specific): A bolt carrier group designed for the .40 S&W cartridge.
- Handguard/Forend: A standard AR-15 handguard or forend. Choose one that fits your preferences.
- Buffer Tube, Spring, and Buffer (PCC-Specific): PCC-specific buffer and spring optimized for the .40 S&W.
- Magazines: Choose magazines compatible with your selected lower receiver. Glock magazines are a popular choice.
- Trigger (Optional Upgrade): A standard AR-15 trigger will function; however, many builders choose to upgrade to a performance trigger.
Assembly Instructions
The assembly process generally follows these steps:
- Lower Receiver Assembly: Install the fire control group, pistol grip, and other lower receiver components.
- Upper Receiver Assembly: Assemble the upper receiver by installing the barrel, handguard, and any other accessories.
- Bolt Installation: Insert the .40 S&W bolt carrier group into the upper receiver.
- Buffer System Installation: Install the buffer tube, spring, and buffer into the lower receiver.
- Upper and Lower Receiver Mating: Connect the upper and lower receivers using the takedown and pivot pins.
- Magazine Compatibility Check: Test the functionality of the magazine by inserting and removing it from the magazine well.
- Function Testing: Perform a thorough function test to ensure the firearm cycles and functions smoothly.
Safety should be your top priority throughout the build process. Follow all manufacturer instructions, use appropriate tools, and wear eye protection. If you’re not familiar with building firearms, seek professional assistance from a qualified gunsmith.
Performance and Practicality: What to Expect
Accuracy
Once assembled, the AR-15 in .40 S&W offers a unique shooting experience. The accuracy of the firearm depends on several factors, including the barrel quality, the ammunition used, and the shooter’s skill. The modularity of the AR-15 platform allows for customization of optics, sights, and other accessories to enhance accuracy. Expect the weapon to be quite accurate at ranges beyond that of a typical handgun.
Range and Effective Range
The effective range depends on the barrel length and ammunition. In a typical AR-15 barrel length, the effective range will often exceed 100 yards, although the .40 S&W cartridge will lose energy more rapidly than a rifle cartridge.
Recoil Management
The .40 S&W round offers a balance of power and manageable recoil. While the recoil impulse will be more pronounced than in a 9mm carbine, the AR-15 platform’s design helps mitigate felt recoil. This results in improved control and follow-up shot speed, allowing for more rapid follow-up shots.
Advantages
The AR-15 in .40 S&W has various advantages. It leverages the AR-15’s modularity, allowing for customization of accessories, like optics, lights, and grips, providing a highly customizable platform. The AR-15’s ergonomics, with its familiar controls and comfortable handling, create an intuitive shooting experience for many. Many accessories from standard AR-15s can be used in this build.
Disadvantages
However, there are disadvantages to keep in mind. Components designed specifically for .40 S&W can be more expensive than standard AR-15 parts. The magazine selection and reliability might be a concern depending on the magazine design and manufacturer. Proper function is heavily dependent on the selected magazine.
Pistol Caliber Carbines: Comparing to Others
When evaluating the AR-15 in .40 S&W, it’s beneficial to compare it to other pistol-caliber carbines. PCCs have gained popularity, offering a compact, affordable, and versatile shooting experience. Other popular PCCs include the SIG MPX and various models from manufacturers like CMMG and Angstadt Arms.
The advantage of the AR-15 platform is its modularity. There are a wide variety of custom parts available, allowing shooters to tailor the weapon to their needs. However, specialized builds like the AR-15 in .40 S&W can be more complex and may involve higher initial costs.
Legality and Compliance: Navigating the Laws
Building or owning an AR-15 in .40 S&W involves adhering to all relevant local, state, and federal laws. Understanding these laws is crucial to prevent unintentional violations and ensure your firearm is legally compliant. Make sure your planned barrel length and overall length meet federal regulations. Also, be sure to know your state’s laws regarding assault weapons and magazine capacity restrictions.
Conclusion
The AR-15 in .40 S&W presents a unique proposition within the ever-evolving world of firearms. It’s a build that combines the proven AR-15 platform with the performance characteristics of the potent .40 S&W cartridge. While the build does involve some challenges, it presents a rewarding project for the dedicated firearm enthusiast. The blend of the AR-15’s modularity, the .40 S&W’s stopping power, and the availability of components opens the door to a specialized firearm. It’s a weapon that provides both an interesting project and a practical firearm. Ultimately, the decision of whether to build an AR-15 in .40 S&W depends on your specific needs, your experience, and your interests. It is a project that requires careful planning, attention to detail, and a commitment to safe practices. However, for those who are willing to undertake the challenge, the outcome can be a highly capable and unique firearm. If you’re a seasoned AR-15 enthusiast looking for a new project or you like the power of .40 S&W, exploring the possibility of building an AR-15 in .40 S&W might be a fascinating experience. Thoroughly researching and educating yourself about the process is the key to success.
