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AR-15 Gas System Troubleshooting

Work the problem from symptom to root cause. Start with the symptom that matches your rifle and follow the checks down. Do the simple checks before the expensive ones.

Symptom

Failure to eject

The spent case is not leaving the ejection port. Sometimes it stays stuck, sometimes it hangs up partially.

→ Check: Is the brass extracting from the chamber but not clearing the port?
Weak or broken ejector spring. Replace the ejector and spring. Common on high round count bolts.
→ Check: Is the brass not extracting from the chamber at all?
Weak extractor spring, worn extractor, or missing extractor insert. Replace the extractor, spring, and insert as a set.
→ Check: Is the bolt not cycling fully to the rear?
Under-gassed rifle. Check gas block alignment over the gas port, check the gas tube for obstructions or misalignment, and verify the gas port is not undersized. A stuck or partially open adjustable gas block produces the same symptom.
→ Check: Is the chamber filthy or rough?
Carbon build up in the chamber can hold brass in place. Clean the chamber with a chamber brush and try again before replacing parts.
Symptom

Excessive recoil or brass ejecting at 12 o'clock or behind

The rifle kicks harder than it should and brass lands forward or behind the shooter.

→ Diagnosis: Over-gassed

Fix in order, cheapest and fastest first:

  1. Install an adjustable gas block and tune it down
  2. Move to a heavier buffer (H to H2 to H3)
  3. Upgrade to a heavier buffer spring (Sprinco Green or A5 rifle spring)
  4. If the rifle is severely over-gassed, verify the gas port size is within spec for the barrel length (oversized ports on budget barrels are common)

An adjustable gas block is almost always the right first move because it addresses the root cause instead of masking it with heavier reciprocating mass.

Symptom

Failure to feed

The bolt cycles but the next round does not chamber properly.

→ Check: Is the bolt stripping a round from the magazine but not chambering it?
Feed ramp issue or magazine. Check the feed ramps for burrs, check that the magazine is fully seated, and try a different magazine. Polishing rough feed ramps with fine emery cloth resolves most cases on budget uppers.
→ Check: Is the bolt not picking up a round from the magazine at all?
Magazine issue or under-gassed. Try a known good magazine first. If the magazine is fine, the carrier is not traveling far enough rearward to clear the top round. Diagnose as under-gassed.
→ Check: Is the round stopping at an angle against the barrel extension?
Magazine feed lip damage, wrong magazine for caliber, or worn magazine spring. Replace the magazine.
Symptom

Double feed

A live round is driven into the base of an already chambered round, or two live rounds are jammed into the action together.

→ Check: Is there a live round behind a chambered round (or spent case)?
Failure to extract the previous case. Extractor issue: worn spring, broken extractor, or insufficient extractor tension. Replace the extractor, spring, and insert.
→ Check: Are there two live rounds in the action?
Magazine issue. Worn feed lips releasing two rounds at once, or a damaged follower. Replace the magazine.
Immediate action.Lock the bolt to the rear, drop the magazine, clear the chamber, then inspect. Never try to force the bolt closed on a double feed.
Symptom

Bolt fails to lock back on empty magazine

The rifle fires every round but the bolt does not stay open when the magazine is empty.

→ Check: Does a fresh, known good magazine also fail to lock the bolt back?
If yes, the rifle is under-gassed or the bolt catch is worn. Single load a round and fire it. If the bolt locks back on a single round but not a full magazine, the bolt catch is likely fine and the carrier velocity is marginal.
→ Check: Does the bolt lock back when you manually pull the charging handle?
If yes, the bolt catch and spring are functional. The problem is carrier velocity. Diagnose as under-gassed: verify adjustable gas block setting, inspect gas tube and gas block alignment, and check for gas key leaks on the carrier.
→ Check: Is the bolt catch itself worn or the detent spring weak?
Replace the bolt catch assembly. This is rare but happens on high round count rifles.
Symptom

Light primer strikes

The rifle goes click instead of bang. Primer shows a shallow dimple.

→ Check: Is this happening on the first shot or only on follow up shots?
First shot: firing pin protrusion or hammer spring issue. Inspect the firing pin for wear and verify the hammer spring is original mil-spec (not a reduced power match spring on a carbine).
→ Check: Follow up shots only?
Bolt bounce from over-gassing. The carrier is slamming home so hard that it bounces off the barrel extension, delaying the hammer reset. Treat as over-gassed: add an adjustable gas block or heavier buffer.
→ Check: Hard primers (military surplus)?
Some 5.56 mil surplus and imported ammunition uses harder primers that a reduced power trigger spring cannot reliably ignite. Swap back to mil-spec hammer spring.

Still stuck?

If you have worked through the decision tree and your rifle still fights you, a qualified gunsmith with a headspace gauge and bore scope can catch problems that are invisible from the shooter's end. Gas port diameter out of spec, barrel extension timing, and bolt lug cracking are all things you want a trained eye on.

Tuning procedureGas system calculatorShop adjustable gas blocksShop buffers
Safety disclaimer.Gas system tuning involves live fire testing. Always follow safe firearms handling practices. If you are unsure about any adjustment, consult a qualified gunsmith.