AR-15 Won't Cycle Right? Here's the First Fix, In Order.
A four-chapter diagnostic walk-through: identify over-gassed vs under-gassed, diagnose failures to eject, fix short-stroking, and match buffer weight to gas system. Worked example uses a PSA 16-inch mid-length 5.56 carbine with a 3.0 oz carbine buffer.
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Most AR-15 cycling problems come down to the same short list of causes. The mistake most people make is swapping parts before they've traced the symptom to its source. This course walks through the four checks that solve the majority of cycling failures, in the order that costs you the least time and money.
The worked example throughout chapters one through three is a PSA 16-inch mid-length 5.56 carbine with a standard 3.0 oz carbine buffer and a fixed gas block. That's the most common configuration people bring to this diagnosis. Chapter four covers buffer weight selection for all configurations, including a short sidebar on what changes when you add a suppressor or run .300 BLK.
Chapter 1 of 4
Over-Gassed vs Under-Gassed: Reading the Symptoms First
Quick Answer
An over-gassed AR-15 ejects brass violently at the 1 to 2 o'clock position, has harsh felt recoil, and may show signs of accelerated bolt carrier group wear. An under-gassed AR-15 fails to lock the bolt back on an empty magazine, short strokes, experiences failures to eject, or has inconsistent cycling.
Before you touch anything on the rifle, read the ejection pattern. Brass tells you everything. Watch where your cases land and where on the clock face they leave the ejection port. That one observation separates over-gassed problems from under-gassed problems, and those two conditions have opposite fixes.
On the PSA 16-inch mid-length with a carbine buffer: brass should land consistently at the 3 to 4 o'clock position, 4 to 8 feet away. Anything significantly different from that tells you something is wrong and which direction it went wrong.
| Symptom | Over-Gassed | Under-Gassed |
|---|---|---|
| Ejection clock position | 1 to 2 o'clock | 4 to 6 o'clock, or stovepipe |
| Ejection distance | 15 or more feet, violent | Weak, dribbles out |
| Recoil character | Sharp, snappy | Soft, then fails to cycle |
| Bolt locks back on empty? | Yes, hard | Sometimes not |
| Brass condition | Dented, torn case rims | Normal |
| BCG wear | Accelerated (cam pin, bolt lugs) | Normal |
Identifying whether your rifle is over-gassed or under-gassed is the first step to solving cycling problems. Running an overgassed rifle long-term causes premature wear on the bolt, cam pin, barrel extension, and buffer. Running an undergassed rifle means unreliable function that can fail you when it matters most. Both conditions are correctable once properly diagnosed.
Ideal gas tuning on the PSA mid-length: brass ejects at 3 to 4 o'clock, lands 4 to 8 feet away, smooth recoil, bolt locks back reliably on every empty magazine.
If over-gassed, the first fix before buying anything is a heavier buffer. Move from the carbine (3.0 oz) to an H buffer (3.8 oz). That resolves mild overgassing on most mid-length builds without touching the gas block. If under-gassed, read the next two chapters before touching anything, because under-gassing is almost always a gas block or gas key problem, not a parts problem.
Chapter 2 of 4
Failure to Eject: Why the Case Stays In and How to Clear It
Quick Answer
The most common causes of failure to eject (stovepipes) are a weak or broken extractor spring, insufficient gas (undergassed), a dirty chamber or bolt, and a worn ejector or ejector spring. Start by cleaning the bolt and chamber thoroughly, then inspect the extractor spring tension and check gas block alignment.
A failure to eject means the spent case is not being thrown clear of the ejection port after extraction. The case gets caught between the bolt and the ejection port, jamming the action. On the PSA mid-length, an FTE early in the rifle's life almost always points to a gas or lubrication problem, not a worn extractor. Work through this checklist in order before replacing parts.
- Step 1: Clean everything first. Carbon on the bolt face, in the chamber, or in the extractor channel is the most common and easiest fix. Clean the bolt, paying close attention to the extractor claw and the area behind it. Clean the chamber with a chamber brush. Lubricate the bolt carrier group properly.
- Step 2: Inspect the extractor. Remove the bolt from the carrier. Push the extractor with your thumb. It should have strong, positive spring tension. If it feels weak or mushy, replace the extractor spring. Check the extractor claw for chips, wear, or damage. The extractor O-ring adds tension. Make sure it is in place and not deteriorated. Upgrade: BCM extractor spring kit (enhanced spring plus O-ring).
- Step 3: Check the ejector. The ejector is the small plunger on the bolt face opposite the extractor. Push it with a small punch. It should have firm spring tension and snap back quickly. A weak ejector spring causes the case to not be kicked out with enough force. Replace the ejector spring if it feels weak.
- Step 4: Evaluate the gas system. If the bolt is not traveling rearward far enough, the ejector does not have enough time or force to kick the case out. Check gas block alignment. Verify the gas key is properly torqued (35 to 40 inch-pounds) and staked. Try a lighter buffer. Look for gas leaks at the gas block and gas key.
- Step 5: Check ammunition. Weak or underpowered ammunition may not generate enough gas pressure. Try a different brand of brass-cased ammunition. Steel-cased ammunition has different extraction characteristics and may cause issues in some rifles.
For new builds experiencing failures to eject, the most likely causes are gas block misalignment (verify alignment during assembly), an improperly staked gas key (gas leak), or a dry bolt (insufficient lubrication). Before blaming components, confirm assembly was done correctly. If building from scratch, a quality BCG with a properly staked gas key prevents most cycling issues on its own.
Chapter 3 of 4
Short-Stroking: When the Bolt Carrier Doesn't Make It All the Way Back
Quick Answer
Short stroking means the bolt carrier is not traveling far enough rearward to pick up the next round or lock back on empty. The most common causes are gas block misalignment, a gas key that is not properly torqued or staked, a buffer that is too heavy, carbon buildup in the gas system, and an undersized gas port for your barrel length and gas system configuration.
Short stroking is the most common cycling malfunction on new AR-15 builds. On the PSA 16-inch mid-length, if you got through the first hundred rounds fine and then started having issues, suspect carbon buildup or a gas key coming loose. If it short-stroked from the first magazine, suspect gas block alignment or gas key staking. Work through this checklist in order.
- Gas block alignment (most common cause on new builds). Remove the handguard and visually inspect the gas block position. The gas block hole must be centered directly over the barrel's gas port. Even 1/32 inch of misalignment significantly reduces gas flow. Use an alignment rod through the gas block hole. It should drop into the barrel's gas port freely. If misaligned, loosen the gas block, reposition, and retighten. For set screw gas blocks, dimple the barrel for positive location.
- Gas key integrity. The gas key on top of the bolt carrier must be torqued to 35 to 40 inch-pounds. The two gas key screws must be properly staked. Material should be visibly pushed over each screw head. A loose gas key leaks gas and dramatically reduces carrier velocity. If the screws are loose, remove, clean, apply Loctite, retorque, and restake.
- Buffer weight. If you upgraded to a heavier buffer without a corresponding increase in gas, the carrier may not have enough energy to cycle fully. Try stepping down one buffer weight: H2 to H, or H to standard carbine (3.0 oz). The correct buffer is the lightest one that cycles reliably without overgassing.
- Carbon buildup. Carbon in the gas tube, gas port, or gas key passage restricts gas flow. Clean the gas key passage with pipe cleaners. Gas tubes that are severely fouled may need replacement. Carbon in the barrel's gas port can be cleared with a drill bit matched to the port size, done carefully.
- Gas port size. Some barrels have conservatively sized gas ports, intentionally, for suppressed or precision use. This may cause issues unsuppressed with weak ammunition. An adjustable gas block does not help with an undersized port. You need more gas, not less. Solution is to open the gas port slightly (specialized work) or change barrels.
In 90 percent of short-stroking cases on new builds, the fix is gas block alignment or gas key staking. Both are assembly issues, not parts issues. Take the upper apart, verify gas block position with an alignment rod, check gas key torque, and reassemble carefully. Do not buy anything until you have confirmed those two things.
Chapter 4 of 4
Buffer Weight: Matching the Damper to Your Gas System
Quick Answer
Start with a standard carbine buffer (3.0 oz) for most builds. Move to an H buffer (3.8 oz) if your rifle is slightly overgassed. Use an H2 buffer (4.6 oz) for suppressed builds or barrels with oversized gas ports. H3 buffers (5.4 oz) are for heavily overgassed or suppressed short-barreled rifles. Rifle-length buffer systems use a 5.0 to 5.6 oz rifle buffer with a rifle-length buffer tube.
Buffer weight directly controls how fast the bolt carrier group cycles. A heavier buffer slows the carrier, reducing felt recoil and bolt speed. A lighter buffer allows faster cycling, which is needed for undergassed setups. Choosing the wrong weight causes either overgassing symptoms (too light) or undergassing symptoms (too heavy).
| Buffer | Weight | Use Case |
|---|---|---|
| Carbine | 3.0 oz | Mid-length on 14.5 to 16-inch barrels, carbine gas on 10.5 to 11.5-inch barrels, .300 BLK subsonic |
| H | 3.8 oz | Carbine gas on 14.5 to 16-inch barrels (tames overgassing), mid-length 16-inch smooth cycling, pistol gas on 10.5-inch 5.56, .300 BLK supersonic |
| H2 | 4.6 to 4.7 oz | Suppressed mid-length builds, overgassed carbine gas on 16-inch, .300 BLK suppressed with supersonic |
| H3 | 5.4 to 5.6 oz | Heavily suppressed short-barreled rifles, extremely overgassed configs |
| Rifle | 5.0 to 5.6 oz | Rifle-length buffer tube only. 18 to 20-inch barrels with rifle-length gas. Not interchangeable with carbine tubes. |
The approach: buy a standard carbine buffer to start. Shoot the rifle and observe the ejection pattern. If brass ejects at 1 to 2 o'clock (overgassed), step up to an H buffer. If brass ejects at 4 to 5 o'clock and the bolt barely locks back (undergassed, or buffer too heavy), step down. Buffer springs also matter. A standard carbine spring works for most builds, but upgraded springs like the Sprinco Blue or JP Enterprises silent captured spring improve the cycling feel and reduce spring noise.
Never use a rifle buffer in a carbine buffer tube. The rifle buffer is longer and will cause the carrier to jam against the closed bolt. Carbine and rifle buffer systems are not interchangeable.
Still Not Cycling Right? Tell Woody What It's Doing.
Work through the four chapters above first. If you have done that and still have a problem, describe your specific rifle and what it's doing and Woody will help you trace it from there.
What's coming next in the First Fix series
- First Fix: Suppressor-Ready Tuning. Adjustable gas blocks, bleed-off vs restriction designs, and dialing in suppressed vs unsuppressed settings without two separate gas block positions.
- First Fix: Buffer Weight Selection Deep-Dive. Buffer spring rate, captured spring systems, and how to build the right buffer stack for your specific gas system, barrel length, and use case.