The function of the LOWER RECEIVER ASSEMBLY is to house the below items. This is also where the main difference between the AR-15 and the M16 is. The fire control group of the M16 allows either 3 round burst or full automatic fire in addition to safe and semi-auto fire. The AR-15 does not, it has only Safe and Semi firing options.
Lower Receiver – The lower receiver is the part of your rifle that is generally considered to be the firearm itself by the ATF. The lower receiver is where you’ll find your weapon’s serial number. This is also the only part that you will need to get from your local FFL.
Fire Control Group – The Fire control group consists of the trigger and the hammer of your AR-15, as well as other necessary housing components.
Receiver Extension and Buffer Assembly – As part of your rifle’s recoil system, this assembly helps absorb a lot of that kick, making a better shooting experience.
Magazine Catch Assembly – to secure the magazine and allow release when finished.
Bolt Catch Assembly – Is there to lock the bolt to the rear either manually or automatically when the last round is fired and the magazine is empty.
Stock – The butt stock is the part of your AR-15 that connects to the rifle’s firing mechanisms.
Pistol Grip – The pistol grip attaches to the lower receiver, giving you a firm handle of your rifle.
Takedown and Pivot Pin Assemblies – The takedown and pivot pins and detents lock the upper and lower receivers together.
The UPPER RECEIVER ASSEMBLY house the below assemblies and are the same for AR-15 and M16 rifles.
Upper Receiver – The upper receiver is the part that contains the bolt carrier group and charging handle. The barrel and the forend are also attached to the upper receiver.
Barrel Assembly – The barrel will play a huge role in your accuracy on the range or in the field. There are many different calibers and lengths available.
Bolt Assembly – Houses the extractor and ejector assemblies. It locks the breech and initiates the ignition of the cartridge
Extractor Assembly – removes the round from the chamber
Ejector Assembly – Ejects the round from the breech
Bolt Carrier Assembly – consists of the firing pin, bolt, cam pin, extractor and gas key. At a very basic level, the bolt carrier group is responsible for loading your rifle, making sure bullets are fired correctly and ejecting spent rounds from the chamber.
Gas System – Your rifle relies on gas pressure to operate in the way that it’s designed. For the most part, gas blocks are installed on the barrel, inside the handguard. The gas tube connects to the block and the upper receiver.
Charging Handle Assembly – A charging handle is the part that pulls your bolt carrier group to the rear when you need to chamber a round or to clear a malfunction.
Forward Assist Assembly – If for whatever reason your bolt isn’t operating properly and won’t close all the way, the forward assist should help make sure it goes back into place.
Ejection Port Cover Assembly – When it’s closed, the ejection port cover will prevent dirt, dust and other debris from dirtying your rifle, as it keeps both the bolt carrier group and the upper receiver clean
Handguard(s) Assembly – The primary uses of rail systems and handguards is to protect your hand from this heat so you can enjoy a comfortable shooting experience.
Sights – Front and rear sights are alined to aim the rifle.
Magazine – You can get different sized magazines, but it’s important to keep in mind that laws for the size of magazines can vary from state to state.
When all of the above are assembled correctly it will allow for the eight cycles of firing:
All of the above parts and systems have many different variants. This is one of the great things about this platform. You can have barrel lengths from 24 to 7.5 inches with each having different twist rates. Handguards, Stocks and triggers also come in a wide variety of options. Bolt on additions seem to be only limited by your wallet.
U.S. Army TM 9-1005-319-10