AR-15 Won't Cycle Right? Here's the First Fix, in Order.
Woody's answers are based on published federal and state law. Not legal advice. Consult a licensed attorney for your specific situation.
Your AR-15 fires but does not cycle right. This walk-through takes you through the four checks that solve most cycling problems, in the order a gunsmith would run them, before you spend a dollar on parts. The running example is a common first rifle: a PSA 16-inch mid-length 5.56 carbine with a standard 3.0 oz carbine buffer. Swap in your own barrel length and buffer as you read.
Chapter 1: Is it over-gassed or under-gassed?
Everything downstream depends on this call, so make it first. Your brass tells you the answer. Fire a magazine through the rifle and watch where the cases land. The gas system either sends too much energy into the bolt carrier or too little, and the two conditions leave completely different fingerprints.
On the PSA 16-inch mid-length with a carbine buffer, a healthy gun throws brass to 3 to 4 o'clock, landing 4 to 8 feet away, with a smooth recoil impulse and a bolt that locks back on an empty magazine. Mid-length gas on a 16-inch barrel gives about 7 inches of dwell, which is the sweet spot for this platform, so if your ejection pattern is off it usually points to a tuning or assembly issue rather than a bad barrel.
| Sign | Over-gassed | Under-gassed |
|---|---|---|
| Ejection direction | 1 to 2 o'clock | Weak dribble at 4 to 6 o'clock, or erratic |
| Ejection distance | Violent, 15+ feet | Barely clears the port, or stovepipes |
| Recoil feel | Harsh, snappy, carrier slams the buffer tube | Soft but unreliable |
| Bolt lock-back on empty | Yes, aggressively | No, or inconsistent |
| Brass condition | Dented cases, torn rims | Normal brass, failed cycling |
Over-gassing beats up your bolt lugs, cam pin, and extractor over time. Under-gassing means the rifle simply cannot be trusted. Once you know which side of the line you are on, go to chapter 2 if you are seeing failures to eject, chapter 3 if the bolt is not traveling far enough, or straight to chapter 4 if the fix is a buffer change. For the full symptom writeup, see the standalone article on over-gassed vs under-gassed symptoms.
Chapter 2: Failure to eject, in diagnostic order
A failure to eject means the spent case came out of the chamber but never cleared the ejection port. The classic version is the stovepipe, with the case trapped vertically between the bolt and the port. It is tempting to buy parts immediately. Do not. Run this sequence first, because the cheapest causes are also the most common.
- Clean everything first. Carbon on the bolt face, in the chamber, or packed into the extractor channel is the most common cause and costs nothing to fix. Scrub the bolt, hit the chamber with a chamber brush, and lubricate the carrier group properly before you touch anything else.
- Inspect the extractor. Pull the bolt out of the carrier and press the extractor claw with your thumb. It should push back with strong, positive tension. If it feels mushy, replace the extractor spring, and check the claw for chips. If your bolt runs an extractor O-ring, confirm it is present and not deteriorated.
- Check the ejector. That small plunger on the bolt face opposite the extractor should snap back firmly when pressed with a punch. A tired ejector spring kicks brass out weakly, which reads as erratic ejection on the chapter 1 chart.
- Evaluate the gas system. If the bolt is not traveling far enough rearward, the case never gets thrown clear. Verify gas block alignment, confirm the gas key is torqued to 35 to 40 inch-pounds and staked, and look for gas leaks at both points. This overlaps with chapter 3, and that is not an accident. Under-gassing shows up as ejection failures before anything else.
- Change the ammunition. Weak or underpowered loads may not build enough port pressure on your setup. On the PSA 16-inch mid-length, quality brass-cased 5.56 should cycle without drama, so if a brand of budget steel case is the only thing choking it, the ammo is your answer.
On a new build, jump straight to step 4. Fresh rifles that stovepipe almost always have a misaligned gas block or an unstaked gas key, not worn parts. The deeper version of this checklist lives in the failure to eject diagnosis article.
Chapter 3: Short-stroking and what actually fixes it
Short-stroking means the bolt carrier does not travel far enough rearward to pick up the next round or lock back on empty. It is the most common cycling malfunction on new builds, and in roughly 90 percent of those cases the cause is one of two assembly mistakes, not a bad part. Work this list in order and stop when the rifle runs.
- Gas block alignment. Pull the handguard and confirm the gas block hole sits directly over the barrel's gas port. Even 1/32 inch of offset chokes gas flow badly. If your gas block uses set screws, dimple the barrel for a positive location once you have it aligned.
- Gas key integrity. The key on top of the carrier must be torqued to 35 to 40 inch-pounds and visibly staked, with metal pushed over each screw head. A loose key leaks gas every shot. If the screws are loose, clean, re-torque, and re-stake.
- Buffer weight. If you upgraded to a heavier buffer without more gas to drive it, step back down one weight. On the PSA 16-inch mid-length, the standard 3.0 oz carbine buffer is the right starting point. The correct buffer is the lightest one that cycles reliably without over-gassing symptoms from chapter 1.
- Carbon buildup. Clean the gas key passage with pipe cleaners. A badly fouled gas tube is easier to replace than to clean.
- Gas port size. Some barrels ship with conservatively small ports for suppressed or precision use. An adjustable gas block cannot help here, because you need more gas, not less. This is the one case where the barrel itself may be the problem.
Notice the pattern: steps 1 and 2 are free, step 3 costs a buffer, and only step 5 costs real money. The complete diagnostic, including how to make an alignment rod check, is in the short-stroking fixes article.
Chapter 4: Matching buffer weight to your gas system
By now you know whether the rifle is over- or under-gassed and you have ruled out assembly problems. The last move is matching the buffer to the gas system, because buffer weight is the cheapest tuning knob on the platform. A heavier buffer slows the carrier and tames over-gassing. A lighter one lets a marginal gas system cycle fully.
| Buffer | Weight | Use it when |
|---|---|---|
| Carbine | 3.0 oz | Mid-length gas on 14.5 to 16-inch barrels, and any setup that already runs clean. This is the baseline in the PSA 16-inch example. |
| H | 3.8 oz | Carbine gas on 14.5 to 16-inch barrels, or a mid-length you want extra smooth. |
| H2 | 4.6 to 4.7 oz | Suppressed mid-length builds and moderately over-gassed rifles. |
| H3 | 5.4 to 5.6 oz | Heavily suppressed short barrels and extreme over-gassing. Rarely needed. |
| Rifle | 5.0 to 5.6 oz | Rifle-length buffer tubes only. Never drop a rifle buffer into a carbine tube. |
The tuning method is simple. Shoot, read the brass against the chapter 1 chart, and move one buffer weight at a time. Brass at 1 to 2 o'clock means step up a weight. Brass dribbling out at 4 to 5 o'clock with a lazy bolt means the buffer is too heavy for the available gas. On the PSA 16-inch mid-length, the factory 3.0 oz carbine buffer usually lands in the healthy zone already, and an H buffer is the common upgrade if the gun feels snappy. Gas system length matters here too: mid-length gas on 16 inches gives about 7 inches of dwell, which is why that pairing is the reference standard. The background math is in the gas system length and buffer weight matching articles.
Still stuck? Tell Woody about your specific rifle.
Every rifle is a specific combination of barrel length, gas system, buffer, ammunition, and assembly history. Describe yours and what it is doing wrong, and Woody will walk the diagnosis with you.