How an AR-15 Bolt Carrier Group Works, in 3D
Watch an AR-15 bolt carrier group unlock, extract and lock again in slow motion 3D, then take it apart part by part. See the cam pin turn the bolt out of the barrel extension, and what each part does.
Quick answer
- The bolt carrier group is the AR-15's action: it feeds, locks, fires, unlocks, extracts and ejects every round, riding back and forth inside the upper.
- The bolt locks with seven lugs behind the barrel extension. Gas pushes the carrier back, and a cam pin in an angled slot turns the bolt 22.5 degrees to unlock it.
- It field strips into five parts with no tools: carrier, bolt, cam pin, firing pin and retaining pin.
Source: the AR-15 technical manual and the GunPartPicker catalog.
See the bolt carrier group work, in 3D
Watch the bolt unlock, extract and lock again in slow motion, or take the group apart part by part. Step through it, scrub it, or drag to turn it.
How it cycles · 1 of 6Locked
How it cycles · step 1 of 6
Locked
The bolt's seven lugs sit behind the barrel extension's lugs, so the bolt can't move back when the round fires. The extractor's claw is over the case rim.
Illustrative, not to scale or real time: the parts are drawn shapes in mil spec proportions, and the gas rings, ejector and extractor spring are left out.
Bolt carrier groups builders pick
Ranked by what builders add most, in stock first. One listing per model.
Bare bolts and parts kits
Ranked by what builders add most, in stock first. One listing per model.
The parts of a bolt carrier group
Five parts come apart by hand, plus the extractor that stays on the bolt.
- Bolt carrier
- The heavy steel body that rides back and forth in the upper. Gas pushes it; its weight carries the cycle through.
- Gas key
- Bolted and staked to the top of the carrier. It takes the gas from the gas tube into the carrier.
- Bolt
- Locks into the barrel extension with its lugs, holds the case head against the pressure, and turns to unlock.
- Cam pin
- Sits through the bolt and rides in a slot in the carrier. As the carrier moves, the slot turns the pin and the bolt with it.
- Firing pin
- Runs through the bolt. The hammer hits its back end and its tip strikes the primer.
- Firing pin retaining pin
- A small cotter pin across the back of the carrier that holds the firing pin in.
- Extractor
- The claw on the bolt's head that grips the case rim and pulls the empty case out of the chamber.
- Barrel extension
- Part of the barrel, not the bolt carrier group: the ring of lugs the bolt locks behind.
How does an AR-15 bolt carrier group work?
On gas and a cam. When a round fires, some of the gas behind the bullet goes up the gas port, back through the gas tube and into the gas key on top of the carrier. It pushes the carrier back. The carrier moves an eighth of an inch on its own, then the angled slot it carries turns the cam pin, and the pin turns the bolt 22.5 degrees so its lugs clear the barrel extension.
From there the bolt and carrier run back together. The extractor pulls the case out, the ejector flips it out of the port, and the carrier rolls the hammer back. The buffer spring then drives everything forward: the bolt strips the next round from the magazine, chambers it, and the carrier's last three eighths of an inch turn the bolt back to locked. See the gas side in DI vs piston and the spring and buffer in the AR-15 buffer system.
Semi auto vs full auto profile carriers
The cut at the back of the carrier, not what it fires.
| Semi auto profile | Full auto (M16) profile | |
|---|---|---|
| Weight | Lighter, by about an ounce | Heavier, about 11.5 oz for the group |
| Back of the carrier | Cut away under the rear | Full length shroud |
| Recoil and reliability | A little faster cycling | Smoother, more forgiving, better suppressed |
| Forced reset triggers | Won't run one | Needed for one |
| Legal | Yes | Yes: the profile alone doesn't make a rifle full auto |
Most carriers sold today are full auto profile, and that is what forced reset triggers need; check one against your rifle in the FRT and binary trigger picker. Compare the most picked carriers in the AR-15 BCG guide.
How do you take apart a bolt carrier group?
By hand, in four moves. Pull the retaining pin out of the left side, drop the firing pin out of the back, turn the cam pin a quarter turn and lift it out, then slide the bolt out of the front. Put it back in reverse, cam pin before firing pin: the firing pin runs through the cam pin and holds it in, so the cam pin has to be there first. Getting the group out of the rifle is shown in upper vs lower receiver.
Frequently Asked Questions
How does an AR-15 bolt carrier group work?
Gas pushes the carrier back, and a cam pin turns the bolt to unlock it. The carrier moves an eighth of an inch alone, its angled slot turns the bolt 22.5 degrees so its lugs clear the barrel extension, then bolt and carrier run back together to extract and eject the case before the buffer spring drives them home.
What does the cam pin do?
It turns the bolt. The pin sits through the bolt and rides in an angled slot in the carrier, so when the carrier moves, the slot turns the pin and the bolt with it, locking or unlocking the lugs.
How many lugs does an AR-15 bolt have?
Seven, plus the extractor in the eighth position. They lock behind eight matching lugs in the barrel extension, which is part of the barrel, not the upper.
What is the difference between a full auto and a semi auto BCG?
The shape of the back of the carrier. A full auto (M16) profile carrier is full length and about an ounce heavier, which cycles more smoothly; a semi auto profile is cut away underneath. Both are legal, and the profile alone does not make a rifle fire automatically.
How do you take apart an AR-15 bolt carrier group?
By hand, with no tools. Pull the firing pin retaining pin, drop out the firing pin, turn the cam pin a quarter turn and lift it out, then slide the bolt out of the front. Reassemble in reverse, cam pin before firing pin.
Is the bolt carrier group part of the upper or the lower?
The upper. It rides inside the upper receiver and comes out the back when the rifle is opened, so any mil spec BCG fits any mil spec 5.56 upper.
Cite this tool: GunPartPicker, “How an AR-15 Bolt Carrier Group Works, in 3D”, https://gunpartpicker.net/tools/how-ar-15-bcg-works. Data from the GunPartPicker catalog and real community builds, prices checked October 29, 2026. Free to cite with attribution.
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