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  • Post last modified:April 16, 2023

This guide covers the basics of AR15 Anatomy and rifle operation. It will cover its anatomy, how it works and how it has evolved in the last decade. 

The AR15 rifle is a popular semi-automatic firearm that has been in production since the 1950s. It was originally designed by Eugene Stoner for ArmaLite, but the rights were later sold to Colt, who introduced it to the military as the M16.

Over the years, the AR15 has evolved to become a highly customizable platform, with a wide range of aftermarket parts, caliber conversions and accessories available. This allows users to tailor the rifle to their specific needs and preferences, whether they're using it for hunting, competition shooting, or home defense.

AR15 Anatomy

ar15 anatomy

AR15 Basic Operation

Charging Handle

The charging handle operates the bolt carrier group and it can do the following:

raptor charging handle
  • Charge the rifle
  • Lock the bolt
  • Release the bolt
  • Clear malfunctions

Fire Selector

ar15 fire selector

The fire selector is responsible for firearm safety, and it can do the following:

  • Switch the rifle to safe (It blocks the trigger bow from moving)
  • Switch the rifle from safe to semi
  • Switch the rifle to full auto / burst (Only on full auto)
  • Switch the rifle to binary (Only for binary trigger)

Trigger System

The AR15 trigger system is simple and cost effective. On a semi auto AR15, the trigger fires one round per trigger pull.


When the trigger resets, the hammer is locked in place for the next trigger pull.

A full auto trigger works similar, but the full auto sear does the magic. The full auto sear drops the hammer as it simultaneously interacts with the bolt carrier group when it returns to battery. This process repeats itself to go full auto.

A burst trigger system works similar, but the sear has a timing gear. The 3rd gear has a bigger notch than the previous notches to catch the hammer.

A binary trigger system allows the shooter to send a round down range with the 1st trigger pull, and the second shot is fired when the trigger is released. This design still falls under the semi auto category based on ATF's definition.

Bolt Carrier Group

The bolt carrier group is the heart of the rifle besides the barrel. It hosts key components that make the rifle cycle and to make the round go off. The BCG can do the following:

ar15 bcg exposed
  • It hosts the firing pin
  • Recycles the rifle
  • The bolt chambers the round
  • Extracts expended casing
  • Engages the full auto sear
ar15 dirty upper receiver
ar15 dirty bcg

This part can get very hot and dirty if not maintained well.

Bolt Catch

ar15 bolt catch

The bolt catch locks the bolt or to release it to chamber a round. The bolt catch is a small but important component to set the state of the rifle. The bolt catch also engage with the magazine follower on empty to lock the bolt.

It's very much needed when clearing malfunction and for safety check.

Magazine Release

The magazine release is a button the user can push to release the magazine. There is nothing fancy there.

Buffer Assembly


The buffer tube houses the recoil spring and the recoil buffer. The recoil spring can do the following:

  • Return the bolt carrier group
  • Different buffer weight for different recoil signature on the AR15
  • Buffer weight can counterbalance suppressed AR15 configurations

Gas System Length

The AR 15 uses direct impingement gas system to cycle the bolt. All it does is send the pressure of gasses in the barrel back to the bolt carrier group to recycle the rifle.

ar15 gas system length

Pistol length gas system -

Carbine length gas system -

Mid length gas system - 

Rifle length gas system - 

Barrel Length 

AR15 barrel length is a big deal. It's the engine of the rifle to send bullets down range. Most modern AR15 barrels are cold hammer forged with chrome lined to prevent corrosion to make them last longer.

The barrel length definitely affects the performance of the bullet. A longer 18" - 20" barrel is always recommended for longer distance shooting. That 2 or 4" extra barrel length makes a huge difference.

In addition, the barrel length and gas system length can be tweaked for particular ammo and optimized suppressor use.

The common barrel length are:

  • 10.3"
  • 14.5"
  • 16" - The most readily available
  • 18"
  • 20"

Twist Rate

AR15 barrel twist rate determines how fast the bullet will spin for a particular length. For example, 1:7 twist means the bullet will spin 1 full revolution per 7" - Faster

AR15 twist rate barrel chart

1:9 twist means the bullet will spin 1 full revolution per 9" - Slower

The twist rate, barrel length, bullet weight, bullet muzzle velocity all have impact on the accuracy of the shot.

Modern AR15 Variants

daniel defense m4a1 eotech exps3 fde

The AR15 has evolved and improved significantly since the M16 era. The rifle functionality is still the Eugen Stoner design, but it has become more modular and adaptive to today's firearm technology.

Modular AR15

Modern AR15 rifles are much more modular than the previous generation. They can adopt more accessories like a LEGO set

Piston Driven AR15 

PWS piston driven MK107 upper in action

Piston driven AR15s significantly reduces heat and carbon residue coming back to the main receiver to increase the service life of the bolt. Rifles like Sig MCX, HK416, BRN 180 are great examples of piston driven AR15.

The gas that supposed to redirect back to recycle the bolt still works the same way, but the action happens at the gas block. A long piston pushes the bolt back.

Bufferless AR15

Sig MCX 11.5

A gas piston AR15 design also present the AR15 the opportunities to remove the fixed buffer tube system with a shorter bolt carrier group.

This allows the rifle have folding stock for something like a PDW (Personal Defensive Weapon)

Side Charging AR15

Side charging AR15 allows the user to charge the rifle in a safer manner by having the rifle closer at hand instead coming all the way back to charge the rifle. This is a very much a hybrid design to make an AR15 works and feel like a SCAR, AK, BRN 180 and others like it.

The entire system is works the same way.

AR15 With Various Improved Ergonomics

There are tons of accessories out here now to improve the ergonomics of the AR15 rifle. These include:

surefire sr07-d-it switch on ddm4v11 left
  • Angled safety selector for faster mode switching using the firing hand thumb
  • PDW angled pistol grip for faster rifle presentation without bending the wrist 
  • Angled foregrip for recoil control
  • Various light activation switches that don't change the shooter's grip position
  • Various sling attachment points and hooks
  • Extended bolt release or ambi bolt release
  • Adjustable buttstock for better cheek positions

AR15 Chambered For Other Calibers


Yes! the AR15 is not only chambered for 5.56 and 223. There are many different caliber types available for the AR15 platform today than previous generation. 

Some of the most popular ones are:

If building an AR15, the user has many options for different applications

picking ar15 ammo

As the AR15 platform has evolved over time, there are many cool designs out there beyond its normal configuration. Some of the most popular ones are the piston driven AR15s that feature AK / FN SCAR like features:

  • Sig MCX
  • BRN 180

Others ones are:

  • Pistol caliber carbine design 
  • Lever action
  • 5.7mm upper receiver with magwell ejection port