AR-15 — Complete Builder's Guide

The lower receiver is the only part of an AR-15 that is legally the firearm in the eyes of the ATF. It carries the serial number, must be transferred through an FFL, and is the foundation every other component attaches to. Understanding the lower is the first step in understanding the platform — once you grasp how it hosts the fire-control group, mag well, and buffer tower, every other AR-15 part starts to make sense.

The upper receiver is where the AR-15's action happens. It hosts the bolt carrier group as it cycles, anchors the barrel through the barrel nut, mounts the handguard, and provides the Picatinny rail for optics. Unlike the lower, the upper is not legally the firearm — you can buy complete uppers online and have them shipped to your door.

The BCG is the heart of the AR-15's operating cycle. Gas tapped from the barrel pushes the carrier rearward; the carrier extracts and ejects the spent case; the buffer spring drives it forward; the bolt strips a fresh round from the magazine and locks into the chamber. All of this happens in about 1/30th of a second.

The charging handle is the only way to manually cycle the action. You use it to load the first round, clear a malfunction, or lock the bolt to the rear for inspection. It looks simple — and the mil-spec version is — but it's also one of the most upgraded parts on a modern AR-15 because of how often your hand touches it.

The barrel is where ammunition meets engineering. Every dimension — length, twist rate, profile, chamber spec, gas system, muzzle threading — affects accuracy, reliability, weight, sound, and legal classification. Getting the barrel right matters more than almost any other choice.

When you pull the trigger, expanding propellant gas in the barrel has two jobs: push the bullet down the bore, and then cycle the action. How the gas does the second job is the most consequential engineering choice in the AR-15 family.

The handguard does three jobs: protects your support hand from the barrel's heat, provides mounting real estate for lights/lasers/sights, and (if it's free-float) prevents your grip from flexing the barrel and ruining accuracy. The handguard market is huge — picking right comes down to mounting standard, length, and weight.

Everything that screws onto the end of your barrel is a muzzle device. Each type makes a deliberate tradeoff: less recoil, less flash, less concussion, or being a suppressor host. Pick based on where and how you shoot.

The trigger is the only interface between you and the rifle's mechanical action. A bad trigger guarantees bad shooting; a good trigger transforms the rifle. It's also the upgrade most shooters feel most immediately — more than optics, barrels, or stocks.

The stock anchors the rifle to your shoulder, and inside it (or in the buffer tube) is the spring + buffer that drives the bolt forward. Length of pull, weight, and adjustability all affect how the rifle shoots — and the buffer weight affects how it cycles.

The pistol grip is the part of the rifle your firing hand grabs. It's small and cheap but disproportionately impacts shooting comfort. It's also one of the 'features' some states use to define an 'assault weapon' — meaning swapping it can affect legal status.

The AR-15 platform isn't married to 5.56 NATO — by swapping the upper (sometimes just the barrel + bolt), the same lower can host 9mm, .22 LR, .300 BLK, 6.5 Grendel, 6mm ARC, .224 Valkyrie, and more. The magazine usually changes with the caliber.