AR15 vs M16: Key Differences You Need to Know

Project ar15 Apr 14, 2024
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Ar15 vs M16

AR15 vs M16. What's the Difference?

Since it's induction into military service the M16 has evolved into the M4 Carbine. Which is essentially a shortened version of the M16 that utilizes a carbine length gas system. So, for the sake keeping information modern and relevant, we will be comparing the Ar-15 to the military's M4.

The AR-15 and the M4 are two firearms that often find themselves in the crosshairs of debate, with enthusiasts and critics alike arguing about their similarities and differences. While some may spin tales of the M4's supposed magical superiority, the truth lies in the details. In this article, we will delve into the specifics that set these two firearms apart, dispelling myths and uncovering the facts.


The M4: Military Designation with a Simple Twist


The M4, a widely recognized firearm, owes its fame to its military designation. However, there is no secret sauce or Un-obtainium material that makes the M4 special. In essence, the M4 is merely a full-automatic or burst-fire capable version of the AR-15. The heart of both these firearms is the AR-15's trigger system, consisting of three main components: the trigger, hammer, and disconnect.


The Third Pin: A Key Difference


What sets the M4 apart is the addition of an extra mechanism within the lower receiver that allows for continuous or burst fire. This crucial addition is held in place with a distinctive pin on the lower receiver, aptly known as "the third pin." This pin retains a secondary sear interface, enabling the selector to dictate trigger reset and operate in automatic or burst modes. Essentially, when the safety selector is set to Burst or Auto the primary sear is disengaged, being pushed down by the selector. Simultaneously the rotating of the selector engages the secondary sear interface. This secondary sear acts as the primary sear does with one key difference: it is tripped by a part of the BCG known as the Second Sear Trip Face. This is the Full-Auto profile of the bolt carrier. This extra material on the BCG runs into the second sear and trips it, subsequently releasing the hammer and firing another round. This process will continue to so long as the trigger is being pulled and ammunition is available via the magazine.

Want more information about Full Auto Bolt Carrier Groups? Check out our article, AR-15 BCG: The Bolt Carrier Group (project-ar15.com)

Unlike the AR15, the M4 is classified as a "machine gun", as are nearly all small arms capable of automatic or burst fire. And therefore subject to regulation and restrictions by the Federal Government. Aside from the ability to fire automatically, this would have to be the biggest difference between the Ar15 and the M4. The Rifle pattern itself, its modularity, components, feeding and gas system are both made to the same mil-spec standard for uniformity.

AR15 Auto Sear Secondary Sear Interface

A Commonality of Gas Systems


While some military service rifles utilize a gas piston system, the majority of both military and civilian AR-15s rely on direct impingement. This commonality between the AR-15 and the military M4 means that the core operating principle remains unchanged.


Automatic vs. Semiautomatic Accuracy and Control


It's important to note that the debate between AR-15 and M4 often centers around their automatic or burst-fire capabilities. However, the truth is that in most scenarios, fully automatic and burst fire modes are employed for specific purposes, such as cover fire, and not for precision shooting. These modes tend to reduce accuracy, consume ammunition rapidly, and can result in errant projectiles.


On the other hand, semiautomatic fire, available on both the AR-15 and the M4, allows for rapid yet controlled shots on target. With proper training and diligence, skilled shooters can achieve impressive accuracy and efficiency.


Is The M4 Better Than The AR15?


The notion that AR-15s are inferior to military M4s is a persistent myth perpetuated by internet "Gandalfs" who may be seeking to ascribe a touch of magic or importance to the military designation. However, when we examine the core components and capabilities of both firearms, we find that they share much in common. With only a small mechanism in the rear of the lower receiver attributing to their difference in functionality. There are no special materials or qualities that make the M4 superior to the Ar-15. The ever-persistent innovation of AR15 components has further blurred the line between the AR15 and M4 Carbine with parts like Forced Reset Triggers. Giving AR15 users a third fire control option for a faster rate of fire potential.



In the world of firearms, the AR-15 and the M4 are not worlds apart. The key differences, such as the addition of a mechanism for automatic or burst fire, the use of a third pin and government classification/regulation do set them apart, but their underlying principles remain similar. Rather than focusing on myths and legends, it's essential to recognize the capabilities and limitations of these firearms accurately. Understanding these nuances ensures that debates about their use and regulation are grounded in facts, not fantasy. And the proliferation of this basic knowledge will help gun owners further understand how the AR-15 and M16 function.