AR-15 Malfunction Diagnosis: Reading What Your Rifle Is Telling You

What This Article Covers

Diagnosing AR-15 malfunctions means identifying root causes, not just clearing stoppages. Understanding what causes a malfunction is what separates a temporary fix from a permanent one. For background on the gas system and buffer system behavior that drives most cycling malfunctions, see understanding gas systems and the buffer system explained.

Before Diagnosing: Isolate Variables First

Wear ballistic-rated eye protection (ANSI Z87.1 or MIL-PRF-32432) and hearing protection (NRR 29+ earplugs or NRR 22+ earmuffs) during any live-fire diagnostic session. Diagnosing an active malfunction often means firing additional rounds to reproduce the condition. Do not skip PPE because the session feels informal.

Before attributing a malfunction to a specific part, establish baseline conditions:

1. Swap to a known-good magazine. Magazines are the most common source of feed malfunctions and are frequently overlooked. A bad magazine mimics BCG, gas, and buffer problems.
2. Clean the rifle thoroughly. Carbon fouling in the chamber, on the bolt face, and in the firing pin channel causes or contributes to many malfunctions that appear to be component failures.
3. Verify lubrication. An under-lubed bolt carrier group increases friction and produces symptoms similar to an under-gassed rifle. Break Free CLP handles both cleaning and lubrication in a single product.

If the malfunction persists after clean conditions and a known-good magazine, proceed to root cause diagnosis.

Failure to Feed

The bolt goes forward but the round does not chamber. This includes partially chambered rounds and bolt-over-base malfunctions (where the bolt rides over the top of the cartridge rather than pushing it into the chamber).

Diagnostic path:

• Magazine first. Damaged feed lips, a worn follower, or a weak spring all cause FTF. Compare malfunction rate between multiple magazines.
• Feed ramps. Carbon or rough machining on the lower feed ramps or barrel extension feed ramps can deflect rounds. Inspect under light and clean with a bronze brush.
• Buffer weight. A buffer that is too heavy slows bolt travel enough that the bolt doesn’t strip the next round cleanly from the magazine. Review the buffer system guide for weight-to-gas-system pairing. The BCM MK2 T1 buffer at 4.7 oz is a reliable carbine-length baseline to compare against.
• Under-gassed system. Insufficient gas pressure means the bolt doesn’t travel far enough rearward to reliably pick up the next round. This appears more frequently with adjustable gas blocks set too restrictively or with suppressor use that changes back-pressure dynamics.

Failure to Fire

The round chambers and the trigger breaks, but the round does not fire. Examine the primer after the failure: the dimple depth and shape contain diagnostic information.

Diagnostic path:

• Light primer strike. A shallow, centered dimple indicates insufficient firing pin energy. Common causes: carbon buildup in the firing pin channel (the channel should be clean and dry, as oil in the channel reduces firing pin velocity), a worn or damaged firing pin, or a fatigued hammer spring.
• Off-center dimple. Suggests the firing pin is not striking the primer squarely. Inspect the bolt face and firing pin for damage or wear.
• No dimple. The firing pin did not reach the primer. Check for a broken firing pin retaining pin, a firing pin that is stuck in the channel, or a trigger group issue where the hammer isn’t following through.
• Ammunition. Hard military primers require more strike energy than commercial brass. If failures are ammo-specific, the ammunition is likely the variable rather than the rifle.

Failure to Extract

The round fired but the spent case remains in the chamber. The bolt attempted to travel rearward but could not remove the case.

Diagnostic path:

• Extractor condition. The extractor lip that engages the case rim wears over time. Inspect it: a rounded lip rather than a sharp edge indicates wear. Replace the extractor and extractor spring together. Quality BCGs like the BCM M16 BCG use staked gas keys and MP-tested bolts, which helps rule out manufacturing defects when diagnosing.
• Extractor spring. A weak extractor spring reduces the force holding the extractor against the case rim during extraction. This is a common wear item on high-round-count rifles and an inexpensive fix.
• Chamber condition. A fouled or rough chamber holds the case in place. Fired brass expands to fill the chamber, and fouling increases friction against that expansion and makes extraction harder. Scrub the chamber with a properly sized bronze brush.
• Over-gassed system. Excessive gas pressure drives the bolt rearward before chamber pressure drops to the point where the case contracts away from the chamber walls. Signs: ejection pattern at 12–1 o’clock (too far forward), case rims torn or showing extractor damage, primers that appear flattened or cratered. See gas system tuning for gas block adjustment.

Failure to Eject

The case extracts from the chamber but does not leave the rifle. This includes stovepipes (case standing upright in the ejection port) and cases that fall back into the action.

Diagnostic path:

• Ejector spring. The ejector lives in the bolt face and provides the lateral force that kicks the case out of the action. A weak ejector spring is the most common cause of ejection failures and can be replaced with the bolt disassembled from the carrier.
• Ejection pattern. Brass landing at 7–9 o’clock (for a right-handed rifle) indicates under-gassing: not enough bolt velocity to throw the case clear of the port. Brass landing at 12–1 o’clock indicates over-gassing. Four to five o’clock is the normal range.
• Underpowered ammunition. Ammunition that generates less gas than the system expects can produce consistent ejection failures. This is most common with subsonic loads, low-powder training ammunition, and pistol-caliber uppers using the wrong buffer.

Double Feed

Two rounds attempt to chamber simultaneously. Requires a dedicated clearance drill to resolve: remove the magazine, lock the bolt back, strip the rounds from the chamber, reinsert the magazine, and release the bolt.

Diagnostic path:

• Failed extraction leading to double feed. This is the most common cause. The bolt stripped the next round from the magazine before extracting the fired case. Resolve the underlying extraction problem.
• Magazine damage. Feed lips that fail to control the second round allow it to present too early. Inspect magazine feed lips for deformation. Discard magazines that cannot be returned to spec.

Reading the Brass

Ejected brass carries diagnostic information beyond the primer strike:

Observation	Possible Cause
Bright ring near case base	Excessive headspace (case stretching). See headspace guide
Torn rim	Over-gassed extraction or worn extractor
Flattened or cratered primer	Excessive pressure: over-gassed or out-of-spec ammunition
Sooty case body	Under-gassed: case not fully supported before pressure drops
Consistent crack at case mouth	Fatigued brass: ammunition issue, not rifle

Systematic Approach Summary

1. Clean the rifle and verify lubrication
2. Test with a known-good magazine
3. Test with a different ammunition lot
4. Inspect extractor, ejector, and firing pin
5. Evaluate ejection pattern and case condition
6. Review gas system and buffer weight pairing
7. Check headspace if extraction failures persist with case deformation

Most persistent malfunctions resolve at steps 1–3. Reserve component replacement for failures that persist past those steps.