AR15 Trigger & Fire Control Group (FCG)

The AR-15 is probably the most popular rifle, having been utilized for self-defense, military use, and hunting. Many companies produce their unique version of the AR-15. It is very versatile and compatible with a wide range of parts.

One of the upgradable parts of the rifle is the AR15 trigger. It is the part that is used to shoot the rifle, and it is made up of a mechanism called the fire control group. The fire control group, or FCG, consists of a trigger, a hammer, a sear, a disconnector, and a hook.

ar15 trigger fire control group fcg
how ar15 trigger fire control group works
AR15 Trigger & FCG Function Diagram

Each of these components plays a vital role in how the firing system operates, and in the article, we will discuss all the parts separately. Where they are placed, what their purpose is, and how they look.

AR15 Trigger

The trigger activates the shooting mechanism of the rifle, which is activated by the shooter’s index finger. The counter force that the trigger provides, the texture of the trigger, its length, and so on are all very important details that contribute to the overall experience with the rifle.

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Frequently the trigger is overlooked as an important component, but a smooth trigger allows for a lighter pull and overall better control while doing the shooting. Another factor that can directly impact accuracy is the distance the trigger travels when being pulled. 

ar15 fcg fire control group
AR15 Trigger, Hammer, and Hook Diagram

A shorter trigger travel means the inside component called the hammer will activate sooner, which helps with accuracy and control. The trigger doesn’t directly impact the rifle’s accuracy, but through these factors, it indirectly affects the weapon.

A good trigger will help you shoot with consistency and higher accuracy simply because you will have a better feel for the gun. If you want a quick and cheap upgrade to your rifle, then replacing the AR15 trigger might be the best solution for instant results.

AR15 Fire Control Group (FCG)

The FCG is the system is made out of these components:

  1. Trigger
  2. Hammer
  3. Sear
  4. Disconnector
  5. Hook.


The trigger is the only part that is visible, and it is activated with the pull of a finger by the shooter. When the trigger gets pulled, it releases the hammer and initiates the shooting mechanism of the AR15. 

ar15 trigger
AR15 Milspec Trigger

There are two factors that should be considered when judging a trigger’s quality: the force needed to pull the trigger and the distance it needs to travel before it activates the shooting mechanism.


The trigger activates the hammer, which is responsible for striking the firing pin and igniting the cartridge’s primer, causing the bullet to be fired outside the barrel and toward the previously determined target.

ar15 hammer
AR15 Milspec Hammer

The two factors to consider when judging the hammer’s quality are the hammer weight which determines the force with which it will strike the ignition pin, and the spring tension, which determines the speed with which the hammer will return to its original position.


The AR15’s sear is the area where the trigger and the hammer meet. It is an important part of the firing mechanism, which holds the hammer in place until the trigger is pulled or pulled again. 

The sear is a small piece of metal within the FCG, and its role is the same in the firing mechanism for any weapon. The sear is also a safety component in the FCG that prevents accidental ignitions.

So called auto sears or swift links are the part modified that make the AR15 fully automatic. These are only allowed for those with the correct paperwork or military operators.


After each shooting sequence, everything has to go back in place. The disconnector’s role is to revert the hammer to its original place to be ready for the following shot.

The way it works is that after the bullet is shot from the AR15, the spring tension reverts the hammer into place, and it is held there by the disconnector. This makes the disconnector a key safety feature.


The AR15’s hook is part of the hammer that the disconnector catches to hold it into place. The shape and dimensions of the hook can impact the trigger pull.

ar15 hook
AR15 Milspec Hook

A smaller radius hook will make for a lighter trigger pull, whereas a bigger one will result in a heavier pull.


The firing of a single bullet is a complex process that involves the implementation of certain laws of physics. For this process to work safely and accurately, the rifle’s components need to be up to standard.

This is also valid for the AR15 trigger, even though it’s an often overlooked component. The way an AR15 FCG works is when the trigger is pulled with the shooter’s finger, and it activates the mechanism inside the rifle. 

The hammer is released onto the firing pin, which ignites the bullet out of the barrel. Then the hammer’s spring tension forces it back into place, where it is held by the disconnector. The part which catches the hammer is called the hook. 

Adequately replacing the FCG has the capacity to instantly improve the shooting experience. 

Written By

Dan M.
Latest posts by Dan M. (see all)
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