America’s Rifle, the AR-15, despite the fact that it’s more than 60 years old, is a fine-tuned machine. If it ain’t broke, don’t fix it, right?
Yet, despite the fact, there’s a good bit of engineering that goes into one of these gas-impingement autoloaders, and many small parts that work together. They are significantly more complex than your single-shot break-action, to say the least.
So, with that said, whether you’ve just bought a new rear-charging rifle or are here because you’re kicking around the possibility of building an AR-15 with a side-charging handle, here are some of the lesser-known parts and what they do.
Gas Key
You may know that the bolt carrier group serves basically as the heart of the rifle, containing the firing pin, spring, ejector and extractor that perform the basic functions of the action – but none of it works without the gas key.
The gas key is a component at the face of the bolt carrier group that serves to capture the gas traveling down the gas tube from the block; pressure builds up, the bolt carrier group unlocks, and the force of the gas on the gas key pushes the BCG to the rear.
Depending on how under- or overgassed your rifle is, the gas key will actually be one of the first parts that wears out on the gun; when it starts to flag, you’ll notice more sluggish cycling from the rifle.
Barrel Nut
The barrel nut is a basic but indispensable piece of hardware that secures the barrel extension to the upper receiver. It can also connect the handguard to the upper receiver.
Forward Assist
If you’re building an AR-15 with a side-charging handle configuration, your build won’t have a forward assist, but the majority of AR-15 rifles have one.
In the standard rear-charging configuration, the charging handle can be used to draw the bolt backwards, but once you hit the bolt release, the charging handle is not effective at manually returning the bolt carrier group to battery if it does not do so under the action of the buffer spring alone.
Therefore, there is a small plunger-like button on the right-hand side of the upper receiver, at an angle behind the BCG, that can be hit to manually return the bolt carrier group to battery if for one reason or another the action does not close fully.
Ejection Port Cover
This is another part that, like the forward assist, you will not have if you intend to build an AR-15 with a side-charging handle. But it is another that is common on AR-15 rifles with rear-charging handles.
Also called a dust cover, the ejection port cover is a small panel or door that covers the bolt carrier group as well as the open action, helping to keep dust, dirt and other debris out of the action of the rifle, and by extension keeping the upper receiver clean.
The ejection port cover must be manually closed, but when you operate the rifle, the cover will open on its own. This is an interesting AR-15 part that actually does not need to be present for the rifle to operate, though it does play a valuable role in keeping the action clean.
If you have a rifle with a rear-charging and an ejection port cover, remember to keep the cover closed when the rifle is not in operation.
Cam Pin
The cam pin is a small but essential component integral to the bolt carrier group. The role of the cam pin, though simple, is vital to proper and safe action operation.
The stock cam pin in your bolt carrier group is responsible for ensuring that the bolt rotates properly during cycling, as well as that it locks and opens properly during the firing sequence. Without the cam pin, the bolt would not lock or unlock at the right time and the rifle’s action would fail to operate as intended.
Note for those of you that intend to build an AR-15 with a side-charging handle with one of our uppers: Our uppers are compatible with any mil-spec bolt carrier group provided you swap out the stock cam pin with one of our enhanced cam pins.
Buffer Tube
The buffer tube is a little known AR-15 part that is absolutely critical to rifle operation. For this reason it is better to refer to the buffer tube as the buffer system, because of how its components, however simple, work together.
The buffer tube attaches to the lower receiver and houses the buffer spring and weights. When the rifle fires and the bolt carrier group moves rearward, it encounters the buffer spring, which helps arrest its rearward momentum and which stores energy necessary to return the BCG to battery.
The bolt carrier group also encounters resistance from the buffer weights, which provide mass that resists the rearward momentum of the BCG. Together, the spring and weights slow and stop the BCG, before the spring releases its potential energy, returning the BCG to battery.
The mass of the weights, and the stiffness of the spring, are two important factors that influence not only felt recoil, but also action time; adjusting the buffer weight can have a significant impact on the fluidity of action operation, as well as on ejection.
Sear
While the hammer is a well-known part of the internals of AR-15 operation, the sear is a lesser-known part. Basically, the sear is a component of the trigger assembly that holds the hammer back until enough pressure is applied to the trigger to allow the sear to release it.
AR-15 rifles have a slightly different sear (and bolt carrier group) than their M16 cousins, which utilize an “auto sear” that allows for select fire, including fully-automatic fire through the depression of the trigger.
Shop AR Uppers and Parts Here
If you’re here because you’re considering building an AR-15 with a side-charging handle, shop our collection of complete uppers as well as AR-15 parts, and if you have any questions about compatibility or assembly, get in touch with us directly.