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- How To Keep Your AR Running, Part 1: AR History
- How to Keep Your AR Running, Part 2: Nomenclature and Cycle of Operation
- How to Keep Your AR Running, Part 3: Limited Technical Inspection
Today we’re going to continue our series on Keeping Your AR Running by looking into the AR nomenclature and Cycle of Operation for a Direct-Impingement Gas System.
We’d like to urge everyone to read our first part of this series on the History of the AR. These articles will all be building blocks, as it’s important to get a solid foundation.
Knowing the different parts of an AR and how they operate is essential to being able to diagnose a problem and ensure you’ve taken the necessary steps to prevent problems from occurring.
To describe the different parts of the AR platform we’re going to break it down by what we refer to as the Upper Receiver Group and Lower Receiver Group.
Check out the embedded YouTube video below for a visual walkthrough we’ve put together of these parts and a simple field strip of the AR you can follow along with.
Upper Receiver Group
- Upper Receiver
- Forward Assist
- Brass Deflector
- Ejection Port Cover
- Barrel Nut and the other components that hold the barrel to the upper receiver
- Barrel Extension (Feed Ramps)
- Flash Suppresor
- Gas Tube or Piston (depending on your AR)
- Sights whether they be fixed or BUIS (back up iron sights)
- Optics (optional)
Inside the upper receiver we have the Charging Handle with latch and the Bolt Carrier Group.
Bolt Carrier Group
- Bolt Carrier Key
- Firing Pin
- Cam Pin
- Gas Rings
Lower Receiver Group
- Buffer Retainer
- Action Spring
- Buffer Tube
- Pistol Grip
- Trigger Guard
- Take Down Pins
- Hammer, Trigger and Disconnector
- Bolt Catch
- Magazine Catch / Magazine Release Button
Cycle of Operation
Now that we’ve gone over the individual parts of the AR, lets examine the Cycle of Operation and how the AR functions.
The first step in the cycle of operation for an AR is to charge the weapon, or “make ready.” This is done by ensuring a loaded magazine is present in the weapon, grabbing the charging handle, pulling it to the rear and releasing.
As the bolt carrier group is pulled to the rear by the charging handle, the action spring is compressed in the buffer tube, causing the buffer to reach its rearmost position inside the buffer tube.
While simultaneously moving rearward, the bolt carrier group rides over the hammer and forces it down into the receiver, compressing the hammer spring and cocking the weapon.
As the bolt carrier group is thrust forward from releasing the charging handle (and the expanding action spring), the face of the bolt picks up a new round that is pushed into its path by the upward thrust of the magazine’s follower and spring.
The bolt’s extractor now grips the new round, which also compresses the ejector and ejector spring as the round is seated in the chamber.
As the bolt carrier enters the last 1/2 inch of its forward movement, the cam pin emerges from the guide channel, moves along the cam track, turns the bolt counter clockwise (22.5 degrees) and locks the bolt into the barrel extension.
The weapon is now locked and ready to be fired. Depressing the trigger thrusts the hammer forward, striking the firing pin. The firing pin makes contact with the primer of the round and ignites the propellant in the cartridge, firing the round.
Firing then forces gas through the barrel carrying the round out and forcing a bit of the gas through the gas port. The gas is directed through the gas tube and into the upper receiver into the awaiting bolt carrier key.
As the gas traveling through the gas tube is directed into the bolt carrier key, it’s channeled into the expansion chamber in the bolt carrier. The gas is now trapped between the rear portion of the expansion chamber and the gas rings on the bolt.
The entire bolt carrier group now moves rearward as the gas expands and is contained by the gas rings. This causes the cam pin to rotate 22.5 degrees, “unlocking” the bolt from the barrel extension.
While the bolt carrier group is moving rearward, the spent cartridge is pulled out of the chamber.
When the spent cartridge continues to move rearward with the bolt carrier group, it’s thrown out of the upper receiver chamber as it clears the ejection port. This is due to the ejector spring pushing on the ejector and the spent cartridge. This pressure from the ejector also causes the extractor to release and kick out the round.
As the bolt carrier group moves rearward it rides over the hammer and forces it down into the receiver, compressing the hammer spring. Because the trigger is still depressed at this point, the hammer is engaged with the disconnector. When the trigger is released or “reset,” the hammer slips from the disconnector and is caught by the nose of the trigger.
The weapon is now ready to fire again.
Here’s a couple of YouTube videos that explain the cycle of operation
Hopefully you’re now familiar with the different parts of the AR system, understand how they work and even how to complete a basic field strip.
in our next article we’ll look at the pros and cons of Direct-Impingement vs. Gas Piston, and get into an LTI or Limited Technical Inspection so you’ll know how to properly assess your AR parts for damage.
Let us know if you have any questions regarding what we’ve covered today, and we look forward to continuing this series!
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Hey, I was watching the video and i was planning on building an AR based on a piston system. I'm very interested in the look of the rifle used in the field stripping video on youtube. is there a possibility if you can tell me the parts and size for the upper barrel receiver used in the ar field strip video?
good job in writing as well...this article is packed with the info I need to a meeting with an arms manufacturer. thanks
One small note, mostly because I am an engineer and we are technical little bstrds. I am pretty sure that during the ejection phase of the operation mentioned, specifically the part about the extractor that it does not move, IE there is no pressure that causes it to release, the round simply pivots at that point, until it comes free of the lip. As I said its a little thing, and I do love the article for showing to people who have eyes that glass over when I try to explain it.
One small note, mostly because I am an engineer and we are technical little bstrds. I am pretty sure that during the ejection phase of the operation mentioned, specifically the part about the extractor that it does not move, IE there is no pressure that causes it to release, the round simply pivots at that point, until it comes free of the lip. As I said its a little thing, and I do love the article for showing to people who have eyes that glass over when I try to explain it. Thanks Jim
Bryan....great article. Helps remind us of the importance of the maintenance and functions of our tools. Now give me a similar article for my M-14 (SOCCOM).......which is my favorite work tool! Sorry, my M-4 takes a backseat.....
You have an amazing AR Bryan. I'm going to build one from scratch. I'm thinking about getting a Spikes Tactical Lower, do you have any words on them?
Good article, another one I'm going to favorite and re-read at least once a week. (yeah, I know, it borders on obsession, but I'd rather know the stuff and not need it than need it and not know it).
Thanks, Bryan. Keep up the good work! Looking forward on more articles regarding the AR platform - Josh
Yes, that would be most excellent. I have just acquired a Gen 1 EBR stock and will be starting a build up. Information would be great.
Thanks Graham, I actually have a lower from Spikes coming right now. Shoot an email through the contact form as I have some information that you'll probably like to hear :)
LOL possibly, but I'd stick with SLIP EWL if I were you :)
Glad you liked the article, hopefully the video we made shows up soon!
LOL possibly, but I'd stick with SLIP EWL if I were you :) Glad you liked the article, hopefully the video we made shows up soon!