M16 and M4 Carbine Principles of Operations. Useful for AR-15 Owners

This article provides the general characteristics, description, available components, and principles of operation for the M4- and M16-series weapons. It provides a general overview of the mechanics and theory of how weapons operate, key terms and definitions related to their functioning, and the physical relationship between the Soldier, the weapon, and the optics/equipment attached to the weapon.


The Army standard service rifle is either the M16-series rifle or M4-series carbine. These weapons are described as a lightweight, 5.56-mm, magazine-fed, gas-operated, air-cooled, shoulder-fired rifle or carbine. The fire in semiautomatic (single-shot), three-round burst, or in automatic mode using a selector lever, depending on the variant. The weapon system has a standardized mounting surface for various optics, pointers, illuminators, and equipment, to secure those items with common mounting and adjustment hardware.

Each service rifle weapon system consists of components, assemblies, subassemblies, and individual parts. Soldiers must be familiar with these items and how they interact during operations.

  • Components are uniquely identifiable groups of fitted parts, pieces, assemblies, or subassemblies that are required and necessary to perform a distinctive function in the operation of the weapon. Components are usually removable in one piece and are considered indivisible for a particular purpose or use.
  • Assemblies are a group of subassemblies and parts that are fitted to perform a specific set of functions during operation, and cannot be used independently for any other purpose.
  • Subassemblies are a group of parts that are fitted to perform a specific set of functions during operation. Subassemblies are compartmentalized to complete a single specific task. They may be grouped with other assemblies, subassemblies, and parts to create a component.
  • Parts are the individual items that perform a function when attached to a subassembly, assembly, or component that serves a specific purpose.

Each weapon consists of two major components: the upper receiver and the lower receiver. These components are described below including their associated assemblies, subassemblies, and parts.



An aluminum receiver helps reduce the overall weight of the rifle/carbine and allows for mounting of equipment and accessories. The upper receiver consists of the following (see figure 2-1):

  • Barrel assembly.
  • Barrel. The bore and chamber of the barrel are chrome-plated to reduce wear and fouling over the life of the weapon.
  • Flash hider or compensator. Located at the end of the barrel, is provided to reduce the signature of the weapon during firing and reduce barrel movement off target during firing.
  • Sling swivel. The attachment hardware for the sling system used to properly carry the weapon.
  • Front sight assembly. Includes an adjustable front sight post that facilitates zeroing the weapon, serves as the forward portion of the iron sight or backup iron sight, and assists with range determination.
  • Adapter rail system (ARS). Provided in varying lengths, depending on the variant applied. Used to attach common aiming devices or accessories.
  • Slip ring. Provides a spring-loaded locking mechanism for the weapon’s handguards.
  • Ejection port. Provides an opening in the upper receiver to allow ammunition or spent casing ejection from the weapon.
  • Ejection port cover. Provides a dust cover for the ejection port, protecting the upper receiver and bolt assembly from foreign objects.
  • Forward assist assembly. Provides a Soldier applied mechanical assist to the bolt assembly during operations.


Upper receiver
Upper Receiver



The lower receiver shown in figure 2-2, on page 2-3, consists of the following components, assemblies, and parts:

  • Trigger assembly. Provides the trigger, pins, springs, and other mechanical components necessary to fire the weapon.
  • Bolt catch. A mechanical lever that can be applied to lock the bolt to the rear by the Soldier, or automatically during the cycle of function when the magazine is empty (see page 2-4).
  • Rifle grip. An ambidextrous pistol-type handle that assists in recoil absorption during firing.
  • Magazine catch assembly. Provides a simple, spring-loaded locking mechanism to secure the magazine within the magazine well. Provides the operator an easy to manipulate, push-to-release textured button to release the magazine from the magazine well during operation.
  • Buttstock assembly. Contains the components necessary for proper shoulder placement of the weapon during all firing positions, returning the bolt assembly to battery, and managing the forces of recoil during operation.
    • The M4-/M4A1-series carbine has a four position collapsible buttstock assembly: Closed, ½ open, ¾ open, and fully-open.
    • M16-series rifles have a fixed buttstock with cleaning kit compartment or an applied modified work order (MWO) collapsible buttstock.
  • Action spring. Provides the stored energy to return the bolt carrier assembly back into battery during operation.
  • Lower receiver extension. Provides space for the action spring and buffer assembly during operation.


Lower Receiver


Additional information on the characteristics and components of the M4-/M4A1- /M16-series weapons can be found in technical manual (TM) 9-1005-319-10. Soldiers will use the technical manual for preventative maintenance checks and services (PMCS), and operation under normal conditions, as well as more detailed information on the principles of operation.

Each variant of the rifle and carbine have subtle capabilities differences. The primary differences are shown in table 2-1, and are specific to the weapon’s selector switch, buttstock, and barrel length.


Model Version Firing Methods Comparison




The cycle of function is the mechanical process a weapon follows during operation. The information provided below is specific to the cycle of function as it pertains specifically to the M4- and M16-series weapons.

The cycle starts when the rifle is ready with the bolt locked to the rear, the chamber is clear, and a magazine inserted into the magazine well with at least one cartridge. From this state, the cycle executes the sequential phases of the cycle of functioning to fire around and prepare the weapon for the next round. The phases of the cycle of function in order are—

  • Feeding.
  • Chambering.
  • Locking.
  • Firing.
  • Unlocking.
  • Extracting.
  • Ejecting.
  • Cocking.

For the weapon to operate correctly, semiautomatic and automatic weapons require a system of operation to complete the cycle of functioning. The M4- and M16-series weapons use a direct impingement gas operating system. This system uses a portion of the high pressure gas from the cartridge being fired to physically move the assemblies and subassemblies in order to complete the cycle of function.

Chambering is the continuing action of the feeding round into the chamber of the weapon.


Chambering example


Locking is the process of creating a mechanical grip between the bolt assembly and chamber with the appropriate amount of headspace (clearance) for safe firing. With the M4- and M16-series weapons, locking takes place simultaneously with the final actions of chambering.

Locking example


Firing is the finite process of initiating the primer detonation of the cartridge and continues through shot-exit of the projectile from the muzzle.

Firing example
Firing Example


Unlocking is the process of releasing the locking lugs on the bolt face from the corresponding recesses on the barrel extension surrounding the chamber area.

Unlocking example
Unlocking Example


Extracting is the removal of the expended cartridge case from the chamber by means of the extractor.

Extraction example
Extraction Example

Ejecting is the removal of the spent cartridge case from the weapon itself.

Ejection example
Ejection Example


Cocking is the process of mechanically positioning the trigger assembly’s parts for firing. The cocking phase completes the full cycle of functioning.

Cocking example
Cocking example


Rifle and Carbine Principles of Operation

Cooling is the process of dissipating heat from the weapon during firing. Although not part of the cycle of functioning, cooling the weapon during firing is critical to ensure the weapon continues to operate efficiently. Firing a round generates heat and pressure within the chamber and bore, which radiates outward through the metal of the barrel.

The temperature generated by the burning of propellant powders is over one thousand degrees Fahrenheit. Some of the heat produced during firing is retained in the chamber, bore, and barrel during firing and poses a significant hazard to the firer.

How this heat is absorbed by the weapon and dissipated or removed, is a function of engineering and design.

Lightweight weapons like the M4 and M16 do not have sufficient mass to withstand thermal stress efficiently. The weapon system must have a means to radiate the heat outward, away from the barrel to allow continuous firing.
There are three methods to reduce the thermal stress on a weapon. The M4- and M16-series of weapons use all three of these methods to varying degrees to cool the chamber, bore, and barrel to facilitate continuous operation. These methods of cooling are—

Radiational cooling – allows for the dissipation of heat into the surrounding cooler air. This is the least efficient means of cooling but is common to most small arms weapons, including the rifle and carbine.
Conduction cooling – occurs when a heated object is in direct physical contact with a cooler object. Conduction cooling on a weapon usually results from high chamber operating temperatures being transferred into surrounding surfaces such as the barrel and receiver of the weapon. The transfer from the chamber to the cooler metals has the net effect of cooling the chamber. Thermal energy is then carried away by other means, such as radiant cooling, from these newly heated surfaces.

Convection cooling – requires the presence of a moving air current. The moving air has a greater potential to carry away heat. The handguards and ARS of the rifle and carbine are designed to facilitate air movement. The heat shield reflects heat energy away from the hand guard and back towards the barrel. The net effect is an updraft that brings the cooler air in from the bottom. This process establishes a convection cycle as heated air is continually replaced by cooler air.

Soldiers should be aware of the principles of the weapon’s cooling methods’ direct effects on their line of sight when viewing a target through an aiming device. Dissipating heat along the length of the barrel can create a mirage effect within the line of sight which can cause a significant error to the true point of aim when using magnified optics.


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