I’m hopeful that members will find this helpful when troubleshooting their AR’s. For the most part this comes straight from DOA FM 3-22.9 Rifle Marksmanship M16A1, M16A2/3, M16A4, and M4 Carbine. I have also included the relevant sections from TM 9-1005-319-23&P on troubleshooting each cycle. The troubleshooting procedure assumes that the rifle and ammunition within proper speciation; ie correct chamber and gas port.
Here is a YouTube video for you visual types.
http://www.youtube.com/watch?v=bJODc5t7SGY
The 8 steps in the cycle are;
1. Feeding
2. Chambering
3. Locking
4. Firing
5. Unlocking
6. Extraction
7. Ejection
8. Cocking
CYCLES OF FUNCTIONING
The soldier must understand the rifle components and the mechanical sequence of events
during the firing cycle. The eight cycles of functioning (feeding, chambering, locking,
firing, unlocking, extracting, ejecting, and cocking) begin after the loaded magazine has
been inserted in the weapon.
1. Feeding (Figure 4-3, page 4-4). As the bolt carrier group moves rearward, it
engages the buffer assembly and compresses the action spring into the lower receiver
extension. When the bolt carrier group clears the top of the magazine, the expansion of
the magazine spring forces the follower and a new round up into the path of the forward
movement of the bolt. The expansion of the action spring sends the buffer assembly and
bolt carrier group forward with enough force to strip a new round from the magazine.
FAILURE TO FEED.
Step 1. Inspect for weak or broken magazine catch spring (2).
Replace magazine catch spring (WP 0021).
Step 2. Inspect for defective magazine catch (1).
Replace magazine catch (WP 0021).
Step 3. Determine if magazine catch (1) is out of adjustment (will not retain magazine).
Refer to TM 9-1005-319-10.
Step 4. Check for short recoil. (Short stroking)
See Malfunction 10.
2. Chambering (Figure 4-4). As the bolt carrier group continues to move forward,
the face of the bolt thrusts the new round into the chamber. At the same time, the
extractor claw grips the rim of the cartridge, and the ejector is compressed.
FAILURE TO CHAMBER.
Step 1. Inspect for weak or broken action spring (3). RIFLE ONLY: Free length should be 11 3/4
in. (29.85 cm) minimum to 13 1/2 in. (34.29 cm) maximum. CARBINE ONLY: Free length
should be 10 1/16 in. (25.56 cm) minimum to 11 1/4 in. (28.58 cm) maximum.
Replace action spring (WP 0021).
Step 2. Check for short recoil. (Short stroking)
See Malfunction 10.
3. Locking (Figure 4-5). As the bolt carrier group moves forward, the bolt is kept in its
most forward position by the bolt cam pin riding in the guide channel in the upper receiver.
Just before the bolt locking lugs make contact with the barrel extension, the bolt cam pin
emerges from the guide channel. The pressure exerted by the contact of the bolt locking lugs
and barrel extension causes the bolt cam pin to move along the cam track (located in the bolt
carrier) in a counterclockwise direction, rotating the bolt locking lugs in line behind the
barrel extension locking lugs. The rifle is ready to fire.
FAILURE TO LOCK.Step 1. Inspect for missing bolt cam pin (4).
Replace bolt cam pin (WP 0011).
Step 2. Inspect for damaged bolt carrier key (5).
Repair slightly dented bolt carrier key. Replace more seriously damaged bolt carrier key
(WP 0013).
Step 3. Inspect for loose screws (6) on bolt carrier key (5).
a. Disassemble and repair (WP 0013).
b. Reassemble using new screws.
Step 4. Inspect for improperly assembled extractor spring assembly (7).
Assemble correctly (WP 0012).
Step 5. Inspect for bent gas tube (8).
a. Adjust to original configuration by bending gas tube in area of handguards.
b. Replace gas tube and check alignment (WP 0015).
Step 6. Inspect for weak or broken action spring (3). RIFLE ONLY: Free length should be 11 3/4
in. (29.85 cm) minimum to 13 1/2 in. (34.29 cm) maximum. CARBINE ONLY: Free length
should be 10 1/16 in. (25.56 cm) minimum to 11 1/4 in. (28.58 cm) maximum.
Replace action spring (WP 0021).
Step 7. Check for short recoil. (Short stroking)
See Malfunction 10.
4. Firing (Figure 4-6, page 4-6). With a round in the chamber, the hammer cocked, and
the selector on SEMI, the firer squeezes the trigger. The trigger rotates on the trigger pin,
depressing the nose of the trigger, and disengaging the notch on the bottom of the hammer.
The hammer spring drives the hammer forward. The hammer strikes the head of the firing
pin, driving the firing pin through the bolt into the primer of the round. When the primer is
struck by the firing pin, it ignites and causes the powder in the cartridge to ignite. The gas
generated by the rapid burning of the powder forces the projectile from the cartridge and
propels it through the barrel. After the projectile has passed the gas port (located on the
upper surface of the barrel under the front sight, Figure 4-5) and before it leaves the barrel,
some gas enters the gas port and moves into the gas tube. The gas tube directs the gas into
the bolt carrier. It passes through the key downward into a space between the rear of the
carrier’s bolt cavity and the rear of the bolt itself. The gas then expands.
The bolt is locked into the barrel extension and unable to move forward, and the carrier is thus forced to the
rear by the expanding gas.
FAILURE TO FIRE.
Step 1. Inspect for carbon buildup in firing pin recess inside bolt assembly.
Remove cartridge extractor and clean recess with pipe cleaner (WP 0045, item 11); see
TM 9-1005-319-10.
Step 2. Inspect for broken hammer (10).
Replace hammer (WP 0021 and WP 0023).
Step 3. Inspect for broken hammer spring (9).
Replace hammer spring (WP 0021 and WP 0023).
Step 4. Check for improper assembly of hammer spring (9).
Assemble properly (WP 0023).
Step 5. (M16A2, M16A4, and M4 Only) Determine if burst cam (12) and/or burst cam spring (11)
is frozen or improperly assembled.
Disassemble, clean, lubricate, and reassemble correctly (WP 0023).
Step 6. Inspect for broken, defective, or missing firing pin retaining pin (13).
Replace firing pin retaining pin (WP 0011).
Step 7. Determine if selector lever (14) is frozen on SAFE position.
Disassemble and clean (WP 0021).
Step 8. Inspect for broken firing pin (15) or for firing pin that does not meet gage protrusion
requirement.
Replace firing pin (WP 0011).
5. Unlocking (Figure 4-7). As the bolt carrier moves to the rear, the bolt cam pin
follows the path of the cam track (located in the bolt carrier). This action causes the cam pin
and bolt assembly to rotate simultaneously until the locking lugs of the bolt are no longer in
line behind the locking lugs of the barrel extension.
FAILURE TO UNLOCK.
Step 1. Inspect for burred locking lugs on bolt assembly (16).
Remove burrs.
Step 2. Inspect for burred lugs (17) on barrel extension.
Remove burrs.
Step 3. Check for short recoil. (Short stroking)
See Malfunction 10.
6. Extracting (Figure 4-8). The bolt carrier group continues to move to the rear. The
extractor (which is attached to the bolt) grips the rim of the cartridge case, holds it firmly
against the face of the bolt, and withdraws the cartridge case from the chamber.
FAILURE TO EXTRACT.
Step 1. Inspect for defective extractor pin (18), cartridge extractor (19), and/or extractor spring
assembly (7).
Replace extractor pin, cartridge extractor, and/or extractor spring assembly (WP 0012).
Step 2. Inspect badly pitted chamber with reflector tool (WP 0039, Figure 23, item 3).
Replace barrel assembly if chamber is badly pitted (WP 0015).
Step 3. Check for short recoil. (Short stroking)
See Malfunction 10.
7. Ejecting (Figure 4-9). With the base of a cartridge case firmly against the face of the
bolt, the ejector and ejector spring are compressed into the bolt body. As the rearward
movement of the bolt carrier group allows the nose of the cartridge case to clear the front of
the ejection port, the cartridge is pushed out by the action of the ejector and spring.
FAILURE TO EJECT.
Step 1. Inspect for broken cartridge ejector (22).
Replace cartridge ejector (WP 0012).
Step 2. Determine if cartridge ejector (22) is stuck in bolt body (20).
Disassemble and clean (WP 0012).
Step 3. Inspect for weak or broken ejector spring (21).
Replace ejector spring (WP 0012).
Step 4. Check for short recoil. (Short stroking)
8. Cocking (Figure 4-10). The rearward movement of the bolt carrier overrides the
hammer, forcing it down into the receiver and compressing the hammer spring, cocking the
hammer in the firing position. The action of the rifle is much faster than human reaction;
therefore, the firer cannot release the trigger fast enough to prevent multiple firing.
FAILURE TO COCK.
Step 1. Inspect for worn or broken trigger nose (24) or trigger spring (25).
Replace trigger (23) or defective trigger spring (WP 0024).
Step 2. Inspect for worn or broken hammer trigger notch (28).
Replace hammer (10) (WP 0021 and WP 0023).
Step 3. Inspect for worn or broken hammer disconnector hook (27).
Replace hammer (10) (WP 0021 and WP 0023).
Step 4. Inspect for worn or broken automatic sear hook (26).
Replace hammer (10) (WP 0021 and WP 0023).
Step 5. (M16A3 and M4A1 have only one disconnector) Inspect for worn or broken
disconnector hooks (29).
Replace defective disconnectors (30) (WP 0021).
Step 6. (M16A3 and M4A1 have only one spring) Inspect for weak, broken, or missing
disconnector springs (34).
Replace disconnector springs (WP 0021).
Step 7. Inspect for worn, broken, or missing automatic sear (33).
Replace automatic sear (WP 0021).
Step 8. Inspect for weak or broken automatic sear spring (32).
Replace automatic sear (33) (WP 0021).
Step 9. Determine if long leg (31) of automatic sear spring (32) is incorrectly assembled in receiver.
Remove automatic sear (33) and install correctly (WP 0021).
Step 10. (M16A2, M16A4, and M4 only) Determine if burst cam (12) or burst cam spring (11) is frozen or improperly assembled.
Disassemble, inspect, clean, lubricate, or replace as required (WP 0021).
Step 11. Check for short recoil. (Short stroking)
See Malfunction 10.
SHORT RECOIL. (Short stroking)
Step 1. Inspect for broken or damaged action spring (3).
Replace action spring (WP 0021).
Step 2. Inspect for unlubricated or dirty action spring (3) and receiver extension.
Clean and lubricate.
Step 3. Inspect for improper gap space or worn, missing, or broken bolt rings (35).
a. Stagger bolt ring gaps (approximately 1/3 turn apart).
b. If worn, broken, or missing, replace bolt rings and stagger gaps (WP 0012).
Step 4. Inspect for carbon build-up or foreign matter in narrow passage of bolt carrier key (5). Clean with CLP (WP 0045, item 9) and a worn bore brush.
Step 5. Inspect for broken or bent gas tube (8).
Adjust by bending in area of handguards or replace gas tube (WP 0015).
Step 6. Determine if gas tube spring pin (37) is missing from front sight (36).
Replace gas tube spring pin (WP 0015).
Step 7. Inspect for partially plugged gas system because of carbon build-up in gas tube (8). Replace gas tube (WP 0015).
Step 8. Inspect for carbon build-up in barrel gas port (38).
Remove carbon build-up by soaking barrel in carbon removing compound (WP 0045, item
8). Use rubber gloves (WP 0045, item 17) with carbon removing compound. Use a bore
small arms cleaning brush (WP 0045, item 4).
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