How Does an M16 Rifle Work

Table of Contents

1. Introduction
2. The Foundation of the Platform
3. The Direct Impingement Gas System
4. The Bolt Carrier Group (BCG)
5. The 8 Steps of the Cycle of Operation
6. Select Fire vs. Semiautomatic
7. Maintenance: Keeping the Engine Running
8. Troubleshooting Common Malfunctions
9. The M16 in the Modern Context
10. Conclusion
11. FAQ

Introduction

Standing on a range with an M16 or its civilian AR-15 counterpart is a foundational experience for any modern shooter. Whether you carried one in the sandbox or you are a prepper building out your primary defensive rifle, understanding the internal mechanics of this platform is not optional. It is the difference between clearing a malfunction in two seconds and being left with an expensive club when you need a firearm. At Crate Club, we believe that gear is only as good as the operator’s knowledge of it. The M16 platform has served the US military for over half a century because of its lightweight design and high-velocity performance. This article breaks down the mechanical cycle, the gas system, and the critical components that make this rifle function. Understanding how an M16 rifle works ensures you can maintain it, troubleshoot it, and rely on it when the stakes are high.

Quick Answer: The M16 is a gas-operated, magazine-fed, air-cooled rifle that uses a direct impingement system. When a round is fired, gas is tapped from the barrel and channeled back into the bolt carrier group to cycle the action.

The Foundation of the Platform

The M16 rifle was born from a desire to move away from the heavy, wood-stocked battle rifles of the early 20th century. Designed by Eugene Stoner in the late 1950s, it utilized aircraft-grade aluminum and synthetic materials to reduce weight. The goal was to provide an infantryman with a weapon that allowed for high volume of fire and manageable recoil.

The rifle is chambered in 5.56x45mm NATO, a cartridge designed for high velocity rather than raw mass. This caliber allows the operator to carry more ammunition while maintaining lethal effectiveness at standard engagement distances. The design is modular, consisting of two primary halves: the upper receiver and the lower receiver. These are held together by two captive pins—the takedown pin and the pivot pin.

For a closer look at the cartridge used by this platform, review this 5.56x45mm NATO caliber guide.

The Upper Receiver

The upper receiver houses the "engine" of the rifle. This includes the barrel, the bolt carrier group (BCG), the charging handle, and the gas system. The barrel is where the projectile is stabilized by rifling. The BCG is the heart of the weapon, responsible for the entire cycle of operation. Without a properly functioning upper, the rifle is non-functional.

The Lower Receiver

The lower receiver is the serialized part of the firearm in the United States. It contains the trigger group, the magazine well, the pistol grip, and the buttstock. The lower also houses the buffer tube and buffer assembly, which use a heavy spring to return the BCG to the forward position after firing. This internal layout is what gives the M16 its iconic "straight-line" recoil path, minimizing muzzle rise.

The Direct Impingement Gas System

The most distinct feature of the M16 is how it uses the energy of the fired cartridge to cycle the action. Most gas-operated rifles use a piston. The M16 uses what is commonly called a direct impingement (DI) system.

When you pull the trigger, the hammer strikes the firing pin, which hits the primer of the cartridge. This ignites the gunpowder, creating a massive expansion of high-pressure gas. This gas pushes the bullet down the barrel. About two-thirds of the way down the barrel, there is a small hole called the gas port.

As the bullet passes this port, a portion of the high-pressure gas is diverted into the gas block. The gas then travels back toward the receiver through a small stainless steel tube called the gas tube. The end of this tube fits into a component on the bolt carrier called the gas key.

For additional context on the platform’s operating principle, see this M16 direct-impingement breakdown.

Field Note: Direct impingement systems are lighter and have fewer moving parts than piston systems, making the rifle more balanced. However, they blow hot gas and carbon directly into the receiver, which means the rifle requires regular cleaning and lubrication to remain reliable.

The Bolt Carrier Group (BCG)

If the gas system is the fuel line, the BCG is the engine block. The BCG consists of several critical parts:

• The Bolt Carrier: The main body that travels back and forth.
• The Bolt: The part that actually locks into the barrel and holds the cartridge.
• The Firing Pin: Strikes the primer.
• The Cam Pin: Rotates the bolt to lock and unlock it.
• The Extractor: A spring-loaded claw that grabs the rim of the spent casing.

The gas enters the gas key and flows into a hollow chamber inside the bolt carrier. As the gas expands inside the carrier, it pushes the carrier backward. Because the bolt is locked into the barrel, the carrier moves first. As it moves, the cam pin travels along a curved path in the carrier, forcing the bolt to rotate and unlock from the barrel extension.

The 8 Steps of the Cycle of Operation

To understand how the M16 works, you must understand the eight steps of the cycle of operation. This cycle happens in a fraction of a second. Every time you fire a round, the rifle completes these steps.

1. Feeding

As the BCG moves forward under the pressure of the buffer spring, the bottom of the bolt catches the top round in the magazine. The magazine spring pushes the rounds upward to ensure one is always ready to be stripped.

2. Chambering

The bolt continues to move forward, pushing the round out of the magazine and up the feed ramps. The round is then shoved into the chamber of the barrel.

3. Locking

Once the round is fully seated in the chamber, the bolt reaches the end of its forward travel. The bolt lugs enter the barrel extension. As the bolt carrier continues to move the last fraction of an inch forward, the cam pin rotates the bolt, locking the lugs behind the barrel extension. The rifle is now in battery and safe to fire.

4. Firing

The operator pulls the trigger, releasing the hammer. The hammer strikes the firing pin, which travels through the center of the bolt to strike the primer. The powder ignites, and the bullet is propelled down the barrel.

5. Unlocking

As the bullet passes the gas port, gas travels back the tube and into the BCG. The pressure pushes the carrier backward. The cam pin rotates the bolt in the opposite direction, disengaging the lugs from the barrel extension.

6. Extracting

As the BCG continues its rearward travel, the extractor claw, which is hooked onto the rim of the spent casing, pulls the empty shell out of the chamber.

7. Ejecting

Inside the face of the bolt is a small, spring-loaded pin called the ejector. As the spent casing clears the chamber and the barrel extension, the ejector pin pushes against the side of the casing. This flips the casing out of the ejection port and away from the rifle.

8. Cocking

As the BCG moves fully to the rear, it pushes the hammer back down. The hammer is caught by the sear or the disconnector, resetting the trigger for the next shot. The cycle is now ready to begin again.

Key Takeaway: The cycle of operation is a continuous loop. If the rifle stops at any stage, you have a malfunction. Diagnosing which step failed (e.g., failure to extract vs. failure to feed) tells you exactly what part of the rifle needs attention.

For a broader mechanical overview of the same platform, explore this full M16 operator’s guide.

Select Fire vs. Semiautomatic

The original M16 and its variants (M16A1, A2, A4) are select-fire weapons. This means they have a fire selector switch that allows the operator to choose between "Safe," "Semi," and either "Auto" or "Burst."

In "Semi" mode, the disconnector catches the hammer after each shot. To fire again, the operator must release the trigger to reset the sear and then pull it again. In "Auto" mode, a component called the auto-sear holds the hammer back until the bolt carrier is fully forward and locked. Once the carrier is in place, the auto-sear is tripped, releasing the hammer automatically as long as the trigger is held down.

Most civilian versions of this rifle, the AR-15, lack the auto-sear and have different internal geometry in the lower receiver to prevent the installation of full-auto parts. Mechanically, however, the cycle of operation remains identical for both the M16 and the AR-15.

Maintenance: Keeping the Engine Running

Because the M16 is a direct impingement rifle, it "poops where it eats." Carbon-rich gases are dumped directly into the receiver. If you do not maintain the rifle, this carbon will eventually harden into a substance often called "carbon crust," which can seize the bolt or prevent the gas rings from sealing.

Critical Cleaning Points:

• The Bolt Tail: Carbon builds up heavily on the back of the bolt. While it doesn't always stop the gun, excessive buildup can cause friction.
• The Gas Rings: There are three rings on the bolt that act like piston rings in a car. They must be staggered so the gaps don't align, ensuring a proper gas seal.
• The Star Chamber: The area where the bolt lugs lock into the barrel extension. If this gets too dirty, the bolt may fail to lock, preventing the rifle from firing.

We often include high-grade cleaning kits and specialized lubrication in our Captain tier crates because we know that a dry rifle is a dead rifle. See what’s included in the Captain tier when evaluating maintenance-focused equipment.

In a survival or tactical situation, your maintenance routine is just as important as your marksmanship.

Field Note: If your rifle starts to sluggishly cycle, the first thing to check isn't the gas tube—it’s lubrication. Adding a few drops of oil to the "weep holes" on the side of the bolt carrier can often bring a dirty rifle back to life in the middle of a course of fire.

For a more detailed maintenance sequence, read this AR-15 cleaning guide.

Troubleshooting Common Malfunctions

No machine is perfect. In a defensive scenario, you need to recognize why the rifle has stopped working. Most M16 malfunctions fall into three categories:

Failure to Feed/Chamber

This is often caused by a bad magazine. If the magazine spring is weak or the "lips" are bent, the bolt might ride over the round or jam it into the side of the receiver.

• The Fix: "Tap, Rack, Bang." Tap the bottom of the magazine, rack the charging handle to clear the bad round, and attempt to fire.

Failure to Extract (The "Double Feed")

This happens when the spent casing stays in the chamber while the bolt tries to feed a new round. It is one of the most stressful jams to clear under fire.

• The Fix: Lock the bolt to the rear, strip the magazine out, cycle the charging handle multiple times to clear the chamber, and re-insert a fresh magazine.

Failure to Fire (The "Click")

If you hear a click instead of a bang, you likely have a "dud" round or a light primer strike.

• The Fix: Rack the charging handle to eject the round and seat a new one. If it happens repeatedly, check your firing pin for damage or your hammer spring for proper orientation.

The M16 in the Modern Context

While the US military has begun the transition to newer platforms like the XM7, the M16/M4 family remains the gold standard for versatility. For the prepper or tactical enthusiast, this platform is the most supported in the world. Parts are everywhere, and the knowledge base is massive.

Understanding the mechanics—how the gas travels, how the bolt rotates, and how the buffer absorbs energy—allows you to customize the rifle to your needs. You can change buffer weights to smooth out the recoil or swap the upper receiver to change the barrel length for different mission profiles.

At Crate Club, we focus on gear that is field-tested and operator-approved. Whether it’s an optic for your flat-top M16A4-style build or a sling that won't fail you on a 10-mile trek, we curate gear that complements this legendary platform. Our Major and General tiers often feature professional-grade accessories designed to withstand the heat and vibration of a high-cadence gas system.

Browse the Gear Shop for individual maintenance, optic, and loadout equipment.

Bottom line: The M16 is a precision machine that uses gas pressure to cycle a complex mechanical sequence; mastering its operation is the first step toward becoming an effective operator.

Conclusion

The M16 rifle is more than just a firearm; it is a masterclass in engineering efficiency. By using the very gas that propels the bullet to also prepare the next shot, it created a lightweight and reliable system that changed the face of modern infantry. For the veteran, it's a familiar tool. For the prepper, it's a reliable protector.

Knowing how the eight steps of the cycle of operation work allows you to move beyond being a "parts changer" and become a true technician of your own equipment. Real readiness comes from the intersection of high-quality gear and the skills to keep that gear running.

If you are looking to build out your kit with gear that has been vetted by Spec Ops veterans, consider choosing a Crate Club subscription. From the essentials in the Lieutenant tier to the professional-grade equipment in the General tier, we provide the tools you need to stay prepared for any scenario.

• Next Step: Practice a full field-strip of your rifle and identify each component mentioned in the cycle of operation.
• Gear Check: Ensure your range bag has a high-quality lubricant and a cleaning kit specifically designed for the 5.56 platform.
• Stay Ready: Review the Major tier’s equipment to see how advanced tactical accessories fit into a prepared loadout.

FAQ

What is the difference between direct impingement and a piston system?

A direct impingement system vents gas through a tube directly into the bolt carrier to cycle the action, while a piston system uses that gas to push a physical rod that then moves the bolt carrier. Direct impingement is lighter and generally more accurate due to fewer moving parts on the barrel, but it runs much dirtier than a piston system.

Can an M16 fire both 5.56 NATO and .223 Remington?

Yes, a rifle chambered in 5.56x45mm NATO can safely fire .223 Remington ammunition. However, you should generally avoid firing 5.56 NATO rounds in a rifle specifically chambered only for .223 Remington, as the 5.56 NATO round generates higher pressures that the .223 chamber may not be designed to handle.

Why does the M16 have a "forward assist" button?

The forward assist is a plunger on the right side of the upper receiver that allows the operator to manually push the bolt carrier forward. This is used if the bolt fails to fully lock into battery due to heavy carbon fouling or a dirty chamber, ensuring the rifle is ready to fire without having to cycle the entire action.

How often should I clean my M16 or AR-15?

For maximum reliability, you should perform a basic cleaning and re-lubrication after every range session or roughly every 300 to 500 rounds. In harsh environments like sand or high humidity, maintenance should be performed daily to prevent corrosion or grit from interfering with the tight tolerances of the bolt carrier group.