How an AR-15 Buffer System Works, in 3D: Buffer, Spring and Bolt Carrier
Watch an AR-15 cycle in slow motion, cut open: gas drives the bolt carrier back, the bolt unlocks and throws the brass, the buffer and spring soak it up and send it home. Swap carbine, H, H2 and H3 buffers, add gas or a suppressor, and see the carrier speed, where the brass lands and when it short strokes.
Quick answer
- An AR-15's buffer and buffer spring sit in the tube behind the receiver. Gas drives the bolt carrier back into them; the spring stores that push and drives the carrier home to chamber the next round.
- Carbine tubes take one length of buffer in four weights: carbine 3.0 oz, H 3.8 oz, H2 4.6 oz and H3 5.4 oz. The heavier ones swap steel weights for tungsten.
- A heavier buffer slows the carrier: brass lands further back, recoil is softer and the cyclic rate drops. Too heavy and the rifle short strokes.
Source: buffer maker specifications; brass positions from this tool's cycle model.
Watch an AR-15 buffer system cycle, in 3D
Press play, or tap a step. Drag to turn it, pinch to zoom, and cut it open to see the carrier, the buffer's weights and the spring. Change the gas or add a can and watch where the brass goes.
H buffer, 3.8 ozTuned · 1000x slower
The bullet passes the gas port. Gas runs back down the gas tube, through the carrier key into the chamber behind the bolt's gas rings, and drives the carrier rearward while the bolt stays locked.
Like a 16" carbine gas barrel.
A can's back pressure pushes the carrier harder.
TunedTuned. Brass lands at 3:30, the carrier cycles fully and stops gently. Leave it alone.
Brass lands at
3:30
3 to 4 is tuned
Carrier speed
15.9 ft/s
modeled, as the gas lets go
Back of the tube
Taps
0.4 J left at impact
One cycle
75 ms
about 790 rounds a minute on full auto
A teaching model, not a measurement: one push of gas, a mil-spec carbine spring, an 11.5 oz carrier group. Real rifles vary with the barrel, the ammo and the spring. Get a starting buffer for your barrel
H buffers in stock
Ranked by what builders add most, in stock first. One listing per model.
What does a buffer do in an AR-15?
When a round fires, gas tapped from the barrel runs back down the gas tube, through the bolt carrier's key and into a small chamber behind the bolt's gas rings, and shoves the carrier rearward. The carrier unlocks the bolt, pulls the empty case out and throws it. The buffer rides against the back of the carrier the whole time, so the two travel back into the tube together against the buffer spring.
The buffer adds its mass to the carrier's. Its weight sets how fast the carrier moves: heavier is slower, so it unlocks a touch later, throws brass further back and hits the back of the tube softer. The spring stores the push and sends everything home, stripping the next round off the magazine and locking the bolt.
The weights inside slide. When the carrier slams shut they keep moving forward and land a moment later, which stops the carrier bouncing back off the barrel extension. That's what the "Lock" step shows in the cutaway.
AR-15 buffer weights: carbine, H, H2 and H3
Carbine, H, H2 and H3 are the same length and fit any carbine tube; only the weights inside change. Silver is steel, dark is tungsten.
| Buffer | Weight | Weights inside | Usually runs in |
|---|---|---|---|
| Carbine buffer | 3.0 oz | 3 steel | 16" mid-length and intermediate gas rifles, and rifles that run a little soft |
| H buffer | 3.8 oz | 1 tungsten, 2 steel | 14.5" and 16" carbine gas rifles, and most rifles as a first try |
| H2 buffer | 4.6 oz | 2 tungsten, 1 steel | Short barrels on pistol or carbine gas, suppressed rifles and forced reset triggers |
| H3 buffer | 5.4 oz | 3 tungsten | Suppressed short barrels and heavily overgassed rifles |
Not sure where to start? The buffer weight calculator gives a starting weight from your gas system, barrel length, tube, ammo and muzzle.
How do I know if my buffer is too light or too heavy?
Read the brass. Fire ten rounds and look where the cases land, with 12 o'clock at the muzzle.
| What you see | What it means | What to do |
|---|---|---|
| Brass at 1 to 2 o'clock, dented cases, sharp recoil | Overgassed. The carrier is moving too fast. | Go one buffer step heavier, or close an adjustable gas block. |
| Brass at 3 to 4 o'clock, consistent pile | Tuned. Leave it alone. | None. |
| Brass at 5 to 6 o'clock, weak ejection, no lock back on empty | Undergassed or overbuffered. | Go one step lighter, check the gas block alignment and gas key screws. |
| Runs fine suppressed, short strokes unsuppressed | Buffer is set for the can. | Adjustable gas block, or accept a lighter buffer and more gas to the face suppressed. |
Carbine vs rifle buffer system
A collapsible stock rides a carbine tube, which takes the short carbine buffer family and the carbine spring. A fixed A2 style stock covers a longer rifle tube with a longer, heavier buffer and a longer spring. The longer rifle spring isn't squeezed as hard over the stroke and the heavier buffer slows the carrier, so the push back is smoother.
The parts don't mix: a rifle buffer won't fit a carbine tube, and a carbine buffer and spring in a rifle tube won't cycle right. The A5 system sits between them, a carbine style tube 0.75 inches longer that takes a rifle spring and its own A5 buffers.
Frequently Asked Questions
How does an AR-15 buffer system work?
Gas from the barrel pushes the bolt carrier back. The carrier unlocks the bolt, extracts and ejects the case, then pushes the buffer back into the buffer tube, compressing the buffer spring. The spring then drives the buffer and carrier forward, the bolt strips a new round off the magazine, chambers it and locks.
What does a heavier buffer do?
It slows the bolt carrier. Brass lands further back (toward 4 o'clock), recoil feels softer, the carrier hits the back of the tube less hard and the cyclic rate drops. Too heavy for the gas the rifle has and it short strokes: weak ejection and no lock back on an empty magazine.
What are H, H2 and H3 buffers?
Heavier versions of the 3.0 ounce carbine buffer, all the same length. Each swaps one of the three steel weights inside for tungsten: H (or H1) 3.8 oz with one tungsten weight, H2 4.6 oz with two, H3 5.4 oz with three.
Why are there loose weights inside a buffer?
So the carrier doesn't bounce. When the carrier slams forward into battery, the weights inside the buffer keep moving and hit a moment later, pushing the carrier back into battery instead of letting it rebound. The cutaway shows them slide.
Does the buffer spring matter as much as the buffer?
Yes. A worn carbine spring lets the carrier run back faster and returns it weaker. A carbine spring has 37 to 39 coils; replace it when it's clearly shorter than a new one beside it, or when the rifle starts acting gassy. Flat wire and extra power springs change the timing like a heavier buffer does.
Can I put any buffer in any buffer tube?
No. Carbine, H, H2 and H3 fit carbine tubes. Rifle buffers fit rifle tubes. A5 buffers fit A5 tubes. Each tube takes its own spring.
Is the 3D cycle a real measurement?
No, it's a teaching model: one push of gas against a mil-spec carrier, buffer and spring, tuned so a 16 inch carbine gas rifle with an H buffer throws brass to about 3:30 and cycles at a rate inside the M4's published 700 to 970 rounds a minute. Its carrier speeds and impact numbers are modeled, not measured. It shows which way each change moves things, not the exact numbers for your rifle.
Cite this tool: GunPartPicker, “How an AR-15 Buffer System Works, in 3D: Buffer, Spring and Bolt Carrier”, https://gunpartpicker.net/tools/ar-15-buffer-system. Buffer weights from maker specifications; the cycle is a teaching model. Free to cite with attribution.
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