Parts of the Firearm: The Bolt Carrier Group

In our previous post, we studied the parts of the gun that comprise the fire control group (a.k.a. the trigger control group). In this post, we will study another group of components, the bolt carrier group (or BCG).

The bolt carrier group is usually found in firearms that have a gas operated action. These are the parts that control extracting the old cartridge, cocking the firearm and loading a new cartridge. There are several parts that comprise the bolt carrier group:

Disassembled parts of the bolt carrier group from an AR-15 rifle

In the above image, we see the main parts comprising the bolt carrier group of an AR-15 rifle. There are seventeen parts that comprise the bolt carrier group for the AR-15. They are:

1. Bolt (A)
2. Ejector (B)
3. Ejector spring (C)
4. Ejector roll pin (D)
5. Extractor (E)
6. Extractor pin (F)
7. Extractor spring (G)
8. 3 gas rings (H)
9. Bolt carrier (I)
10. Bolt cam pin (J)
11. Bolt carrier key (K)
12. 2 Bolt carrier key screws (L). These attach the bolt carrier key to the bolt carrier.
13. Firing pin (M)
14. Firing pin retainer pin (N)

There are eight basic operations that are done by the bolt group on an AR-15 or M-16:

1. First, the bolt is in its rearmost position. The action spring then pushes the bolt forward and as it moves forward, it picks up a bullet from the magazine and pushes it towards the chamber via the feed ring.
2. As the bolt carrier moves forward, the bolt passes through cuts in the barrel extension and a cam pin causes the bolt to rotate, so that the locking lugs on the bolt are locked as the bolt reaches its forward most point.
3. When the user pulls the trigger, the sear releases the hammer (we covered these parts in our last post about the trigger group). The hammer spring then rotates the hammer with force into the back of the firing pin (which is part of the bolt carrier group). The firing pin passes through a hole in the middle of the bolt carrier and the bolt and the other pointy end of the firing pin strikes the primer of the cartridge, thereby firing the weapon.
4. Fourth, as the bullet leaves the barrel, some of the gases behind it are tapped into a gas tube. The hot gases travel down the tube, down through the bolt carrier key and are redirected forward, pushing the gas rings on the bolt. This pushes the bolt forward slightly and the bolt carrier to the rear. The rearward movement of the bolt carrier pushes against the cam pin that caused the bolt to lock in step 2. This cam pin now causes the bolt to rotate in the opposite direction and unlock the lugs that were locked in step 2. The bolt is now free to move backwards.
5. As the bolt carrier group moves to the rear, the extractor removes the old cartridge case from the firing chamber and pulls it backwards.
6. As the bolt carrier continues to move backwards, it re-cocks the hammer.
7. As the bolt moving backwards goes past the ejection port, an ejection spring forces the now empty cartridge case to be pushed clear of the extractor and out of the ejection port.
8. Once the bolt has reached backwards to its rearmost point, the action spring pushes the bolt forward again, as described in step 1 and the entire cycle repeats.

The following video gives a decent animation of how things work:

As with the fire control group, it is possible to purchase the parts individually or purchase an entire pre-assembled bolt carrier group part which is ready to be dropped into a firearm. There are aftermarket bolt carrier groups that are plated with hard chrome or titanium nitride for better lubrication and reliability. Some are heavier for slower cycling and others are lighter for reduced loads and faster cycling. Still others are built to much more precise tolerances for consistent locking etc. There is a large selection for users to choose from, depending upon needs and preferences.