What Are The Components Of An AR Build Kit?

What Are The Components Of An AR Build Kit

An AR build kit is an assemblage of parts necessary to build a functional AR-style rifle. These kits, like 3CR Tactical’s AR kits, have gained popularity among firearm enthusiasts for their versatility and personalization. Understanding the components of these kits is crucial for novice and experienced builders. This article will detail each component found in an AR build kit, highlighting their functions and importance in the assembly process.

Lower Receiver

The lower receiver is the foundation of an AR build kit. The part of the rifle is legally considered the firearm and often requires a background check for purchase. Lower receivers can be forged or billet and are typically made of aluminum. They house the fire control group (trigger, hammer, and safety), magazine well, and buttstock attachment point.

Upper Receiver 

The upper receiver is the part that completes the AR rifle, housing the barrel, bolt carrier group, and charging handle. It attaches to the lower receiver and is crucial for the rifle’s cycling action. Upper receivers come in various styles and can include features like forward assists and dust covers. The upper receiver often comes pre-assembled in AR kits with a barrel and handguard. Still, builders can customize these components to suit their specific needs, whether for target shooting, hunting, or tactical purposes.


The barrel is a critical component for the rifle’s accuracy and range. AR kit barrels come in various lengths, materials, and calibers to suit different shooting applications. Shorter barrels offer maneuverability for close-range engagements, while longer barrels provide greater accuracy at longer distances. The barrel’s material, like stainless steel or chrome-lined, affects its durability and performance. The barrel must match the upper receiver’s caliber and gas system length in a build kit. 

Bolt Carrier Group (BCG) 

The Bolt Carrier Group (BCG) is the heart of the AR rifle’s functionality. It consists of the bolt, carrier, gas key, firing pin, cam pin, and retaining pin. The BCG works in tandem with the gas system to cycle the rifle, ejecting spent casings and chambering new rounds. Quality BCGs ensure reliability and longevity. They come in different coatings, such as phosphate or nitride, which affect their resistance to wear and ease of cleaning.

Gas System

The gas system is responsible for the semi-automatic operation of an AR rifle. It includes the gas block and gas tube, which channel gas from the fired round back into the upper receiver to cycle the BCG. The length of the gas system varies based on the barrel length and directly affects the rifle’s recoil and cycling speed. Carbine, mid-length, and rifle-length gas systems are common choices in AR build kits. 


The handguard in an AR build kit serves multiple purposes: protecting the shooter’s hand from the barrel’s heat, providing a platform for attaching accessories, and contributing to the rifle’s overall aesthetics. Handguards vary in length, material, and mounting system. Some feature Picatinny rails for mounting sights, lights, and grips, while others use M-LOK or KeyMod systems for a more streamlined profile. 

Trigger Assembly

The trigger assembly in an AR build kit is a key component for the firing mechanism. It includes the trigger, hammer, disconnector, and associated springs and pins. Trigger assemblies vary in pull weight, crispness, and reset length, affecting the rifle’s overall shooting experience. Builders can choose from standard mil-spec triggers to more advanced options like two-stage or adjustable triggers. AR kits often provide a range of trigger options, allowing builders to choose one that best fits their shooting style and requirements.

Stock and Buffer System 

The stock and buffer system plays a crucial role in the ergonomics and recoil management of the AR rifle. The stock, which attaches to the rear of the lower receiver, can be fixed or adjustable, impacting the rifle’s length of pull and overall comfort. The buffer system housed within the stock includes the buffer tube, buffer, and buffer spring. This system helps absorb recoil and ensures smooth cycling of the BCG.

Sights and Optics

While some AR build kits may not include sights or optics, they are essential for accurate shooting. Iron sights, red dot sights, and scopes are common choices, depending on the rifle’s intended use. Iron sights offer a reliable, no-power-needed option, while electronic sights and scopes provide easier target acquisition and greater accuracy at range.

Accessories and Add-ons 

The final components of an AR build kit are the various accessories and add-ons that enhance the rifle’s functionality and user experience. This can include grips, slings, magazines, and more. These accessories add to the rifle’s capabilities and allow for further personalization. Builders can choose from a wide range of accessories to suit their needs, whether for comfort, convenience, or performance. 

The components of an AR build kit, such as those offered by 3CR Tactical’s AR kits, provide a comprehensive foundation for assembling a customized and high-performing AR rifle. Each component, from the lower receiver to the accessories, plays a vital role in the rifle’s operation, aesthetics, and ergonomics. Understanding these parts’ functions is key to successfully building and customizing an AR rifle. Whether you’re a seasoned builder or new to the world of AR rifles, these kits offer the opportunity to create a firearm that is uniquely yours, tailored to your preferences and requirements.


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