Your AR-15 has a lot of parts, especially compared to more simplistic breech-loading designs, and certainly more than the front-stuffers of yore.
But there are a whole lot of other AR-15 parts beyond the “lock, stock and barrel,” and even the two receivers, that you’re probably familiar with. Here are some of the relatively obscure parts, and what they do.
- Barrel nut: An AR-15’s barrel nut is a special nut that engages a flange on the barrel, securing it to the upper receiver. Properly installed, and with a free-floated handguard, the barrel may then be free-floated for superior accuracy.
- Sear: You’re familiar with the trigger and firing pin, right? But are you familiar with the sear? The sear is a part of the trigger assembly that holds the hammer back until the trigger is pulled, releasing the hammer and setting off the sequence that culminates in the firing pin striking the cartridge’s primer.
- Cam pin: The cam pin is a little-known bolt carrier group part that rotates the bolt so that it locks in the barrel extension. It helps lock the bolt in place, protecting the operator from exposure to hot, high pressure gases.
- Gas key: The gas key is an integral part of the bolt carrier group that receives gasses siphoned off from the gas tube. As pressure builds and the bolt carrier group unlocks, these gasses force the bolt carrier group rearward, assisting with the action cycling sequence.
- Gas block: The gas block is the part that actually clamps onto the barrel, over the gas port, and communicates with the gas tube. Some gas blocks are fixed; others are low-profile to accommodate slim handguards. Others are adjustable, enabling you to adjust how much gas is utilized to siphon the action, enabling you to tailor a rifle that’s either over or undergassed, which can impact extraction and feeding.
- Extractor: The extractor is a small, claw-like part of the forward part of the bolt carrier group that hooks into the rim of the cartridge when the BCG locks forward. It pulls the cartridge free of the chamber when the bolt carrier moves to the rear, either manually or under operation from the gas system.
- Ejector: The ejector is another part of the action that forces the cartridges free of the action, though the ejection port, when the rifle is fired. Without a functioning ejector, the action would jam frequently, even with a working extractor, because the cartridges would not clear the action.
- Bolt carrier door: Present on some uppers, such as the side-charging AR-15 uppers produced by Gibbz Arms, the bolt carrier door is a part that helps retain the bolt, but which also makes it much easier to remove the bolt carrier from the upper for the purposes of disassembly, maintenance, or customization.
- Forward assist: Common on rear-charging uppers, the forward assist is basically a plunger-like part that enables you to manually return the bolt carrier group to battery if it does not fully close for any reason.
- Takedown pins: The takedown pins secure the upper to the lower.
- Buffer tube (and system): Unless you play with the buffer spring and weights, this relatively unknown system might fly under the radar since it’s hidden in your stock. The bugger tube contains the buffer weights and spring which store energy from the bolt carrier group when it cycles rearward, pushing the BCG back forward into battery at the end of the sequence.
Keep This for AR-15 Reference
Store this short post somewhere useful if you ever need to refer back to it for the purposes of some of these relatively obscure AR parts.
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