How to Field Strip & Clean the M4/M16
A clean weapon is a boring weapon, and boring is exactly what you want on the line. Field-stripping the M4/M16 is a five-minute job you will do a thousand times — the trick is doing it the same safe, correct way every single time, including the times you are exhausted and just want to go to bed. Clear it first. Always. Then break it down, clean it, lube the right points, and put it back together so it works when it has to.
You do not begin disassembly on a weapon you have not personally cleared. Not the armorer, not your buddy — you. Treat every weapon as loaded until you have proven to yourself, by eye and by finger, that it is not.
- 1.Point it in a safe direction. Muzzle downrange or into a clearing barrel. Finger straight and off the trigger. Every step below happens with the weapon pointed somewhere a negligent discharge would not hurt anyone. This is not a formality — it is the whole point.
- 2.Try to put the selector on SAFE. If the weapon is not cocked, the selector will not rotate all the way to SAFE yet. That is fine — you will set it after you lock the bolt back. Do not force it.
- 3.Remove the magazine. Press the magazine release, pull the magazine out, and set it aside. A weapon with a magazine still in it is not cleared, no matter what the chamber looks like.
- 4.Lock the bolt to the rear. Pull the charging handle all the way back. While holding it back, press the bottom of the bolt catch so the bolt locks open. Ride the charging handle forward and leave the bolt locked to the rear.
- 5.Now put the selector on SAFE. With the bolt back the hammer is cocked, so the selector rotates to SAFE. Set it there and leave it there for the rest of the process.
- 6.Inspect the chamber — eyes AND finger. Look into the chamber and the upper receiver and confirm there is no round. Then physically check with a finger. Do both, every single time. This is the step people skip when they are tired, and it is the step that gets someone hurt.
What you need
No armorer tools and no punches for a field-strip — that is the whole point of the design. You need a cleaning kit and a place to not lose small parts:
- CLP (cleaner, lubricant, preservative)
- Bore brush and chamber brush
- Cleaning patches and a rod or pull-through
- A small brush for carbon
- A clean surface so the retaining pin cannot escape into the dirt
Field strip: break it into major groups
The M4/M16 field-strips into a handful of groups: the upper receiver assembly, the lower receiver assembly, the bolt carrier group, the charging handle, and the buffer with its action spring. Weapon on SAFE, still cleared, the whole time.
- 01
Push the rear takedown pin
Push the rear takedown pin (the one nearest the buttstock) from the left side. It only travels so far — it stays captive in the receiver, it does not come all the way out. The upper will now pivot open on the front pin.
- 02
Push the front pivot pin and separate upper from lower
Push the front pivot pin the same way and swing the upper receiver fully open, then lift it off the lower. You now have two major groups: upper receiver assembly and lower receiver assembly.
- 03
Remove the bolt carrier group and charging handle
Pull the charging handle back a short way. The bolt carrier group (BCG) will follow it out the rear of the upper — slide the BCG out into your hand. Then pull the charging handle down and back to clear its notch and remove it. Order matters: BCG first, then charging handle.
- 04
Remove the buffer and action spring from the lower
In the lower receiver, press the buffer retainer (the small plunger at the front of the buffer tube) with a fingertip, then let the buffer and action (recoil) spring ease forward and out. Control it — the spring is under tension and will launch the buffer if you just let go.
Break down the bolt carrier group
The bolt carrier group (BCG) is where the carbon collects and where the small, losable parts live. Take it apart over a table, not over the motor pool gravel.
- Carrier (the body)
- Bolt (with extractor and ejector)
- Cam pin
- Firing pin
- Firing pin retaining pin (the cotter pin)
- Gas key (stays on the carrier)
- 01
Pull the firing pin retaining pin
The firing pin retaining pin is the small cotter-style pin through the side of the carrier. Pull it out with your fingers or the tip of a bullet/multitool. This is the part that pings off into the grass and disappears forever — cup your hand around it. It is one of the most-lost small parts on the weapon.
- 02
Drop the firing pin out the back
With the retaining pin removed, tip the carrier nose-down and the firing pin slides out the rear. Do not pry it — if it does not fall free, the cam pin is still locking it in place (next step).
- 03
Rotate and lift out the cam pin
Rotate the bolt cam pin a quarter turn (90 degrees) and lift it straight up and out. If the firing pin would not drop in the previous step, it is because you have to remove the cam pin first — the cam pin physically blocks the firing pin. This is the interlock that stops you from reassembling it wrong, if you pay attention to it.
- 04
Pull the bolt out of the carrier
With the cam pin gone, the bolt slides straight forward out of the front of the carrier. Note how it sits — the extractor (the hooked claw) and its position are what you match on the way back in. Leave the extractor and ejector installed unless you have a specific reason and the tools to service them; they are spring-loaded and easy to lose or mis-seat.
Clean each group
CLP does the work — apply, let it soak, scrub, wipe. You are cleaning for function, not for a white-glove inspection photo.
Bore and chamber
Run a bore brush wet with CLP through the barrel from chamber to muzzle, then patches until they come out mostly clean. Use the chamber brush in the chamber and the locking lug recesses. Do not obsess over a perfectly white patch — a light film is fine and expected.
Bolt carrier group
This is where the carbon lives. Scrub the tail of the bolt, the locking lugs, the gas rings, and inside the carrier and the gas key with CLP and a brush. The bolt does NOT need to be bare, shiny metal — baked-on carbon on the bolt tail is normal and trying to scrape it down to bright steel just wears the part. Clean it, do not destroy it.
Firing pin, cam pin, retaining pin
Wipe the firing pin clean — carbon in its recess is a common cause of light strikes. Clean the cam pin and the retaining pin and confirm the retaining pin is not bent or spread open.
Charging handle
Wipe it down, clean the latch, and clear grit out of the grooves it rides in. It gets ignored and then binds at the worst time.
Upper and lower receivers
Wipe out the upper receiver bore and the locking-lug area. In the lower, clean the trigger group area, the buffer tube, and the pin channels. Compressed air or a brush gets the fire-control grit that a patch cannot reach.
Lubricate the right points
The rule that keeps the weapon running: wet where it moves, light film everywhere else. The bolt carrier group runs wet. Everything else gets a film. More is not better.
Bolt carrier group — generously. The BCG runs wet. Coat the bolt (locking lugs, gas rings, cam pin and its slot), the slide surfaces of the carrier, and the cam pin generously with CLP. This is the one place you want to see it glistening.
Cam pin and firing pin. Light film of CLP on the cam pin and firing pin before they go back in. Enough to wet the metal, not enough to pool.
Charging handle. Light coat along the rails it rides in.
Takedown and pivot pins. A drop on each so they push and pull cleanly.
Bore and chamber — light. A light film only. A bore swimming in CLP just burns off and smokes on the first magazine.
Reassemble in reverse
Reverse the order you took it apart. The BCG goes back together in a specific sequence, and the weapon will physically fight you if you get it wrong — that is a feature, not an annoyance.
- 1.Bolt into the front of the carrier (extractor oriented the way it came out).
- 2.Cam pin in, then rotate it a quarter turn to lock — this MUST go in before the firing pin.
- 3.Firing pin in from the rear. If it will not seat, the cam pin is not installed or not rotated. Do not force it.
- 4.Firing pin retaining pin back through the carrier. If it will not go through, the firing pin is not fully seated.
- 5.Buffer and action spring back into the lower until the buffer retainer catches and holds them.
- 6.Charging handle into the upper first, then slide the BCG in behind it and seat both forward.
- 7.Close the upper onto the lower and push the pivot and takedown pins home.
Function check — every time
Reassembled is not the same as working. A function check takes fifteen seconds and is how you find out on the bench instead of on the range.
- 01
Confirm it is clear — again
Before any function check: magazine out, chamber empty, verified by eye and finger. You just reassembled it; treat it as if it could be loaded.
- 02
SAFE — trigger should not release the hammer
Charge the weapon, put the selector on SAFE, and pull the trigger. The hammer should NOT fall. If it does, stop — the weapon is not functioning correctly and goes to the armorer.
- 03
SEMI — hammer falls, disconnector catches
Move the selector to SEMI and pull the trigger — the hammer falls. Hold the trigger to the rear, charge the weapon again, then slowly release the trigger. You should feel/hear the disconnector reset with a click. Pull the trigger once more; the hammer should fall.
- 04
BURST or AUTO — per your variant
Move the selector to the third position (BURST on an M4/M16A2/A4, AUTO on an M4A1/M16A1) and confirm the trigger releases the hammer as designed for that setting. Then clear it and put it back on SAFE.
The mistakes everyone makes at least once
Not clearing it first
Every negligent discharge story starts with someone who "knew" it was empty. Clearing is the first step for a reason. Magazine out, bolt back, eyes and finger in the chamber — before anything else touches the weapon.
Losing the firing pin retaining pin
That little cotter pin loves to launch itself into the dirt the second it clears the carrier. Pull it over a table or a cupped hand, not over grass at 0500. A BCG missing its retaining pin is a non-functional weapon.
Reassembling the BCG backwards or out of order
The cam pin goes in before the firing pin — the cam pin physically blocks the firing pin, and that interlock is telling you something. If the firing pin will not seat, you skipped the cam pin. If the retaining pin will not go through, the firing pin is not fully seated. The weapon is trying to stop you from getting it wrong. Listen to it.
Drowning it in lube
Wet the BCG, light film everywhere else. Over-lubing is not "extra safe" — in sand and dust it turns your action into a grinding paste that collects grit and jams. The right amount is a discipline, not a generosity.
Chasing the "dry" carbon myth
The bolt tail does not need to be bright metal, and the manual never says it does. Baked-on carbon there is normal. Scraping the bolt down to shiny steel every cleaning wears the part faster than any amount of firing. Clean it functional, not cosmetic.
Key takeaways
- Clear the weapon first — magazine out, bolt to the rear, chamber checked by eye AND finger — before you strip anything. Non-negotiable.
- Field-strip is into major groups: upper receiver, lower receiver, bolt carrier group, charging handle, and the buffer/action spring. No punches or armorer tools required.
- Cam pin goes in before the firing pin. That interlock is what keeps you from reassembling the BCG wrong — if a part will not seat, back up a step, do not force it.
- Run the bolt carrier group wet; light film everywhere else. Over-lubing collects grit and jams the weapon in a dusty environment.
- You do not need bare, shiny metal on the bolt. Carbon on the bolt tail is normal — clean it functional, not cosmetic.
- Finish every cleaning with a function check (SAFE / SEMI / third position) and end on SAFE. If the hammer falls on SAFE, it goes to the armorer.
Sources
- TM 9-1005-319-10 — Operator's Manual, Rifle, 5.56-MM, M16A2/A3/A4 and Carbine, 5.56-MM, M4/M4A1 — The public operator technical manual — clearing procedures, disassembly and assembly, cleaning, lubrication points, and the operator-level function check.
- TC 3-22.9 — Rifle and Carbine — U.S. Army training circular covering weapon handling, clearing, function check, and maintenance standards for the M4/M16 family.
Educational reference grounded in publicly published U.S. military doctrine. Weapon handling and maintenance basics are widely published and taught in basic training — this page contains no operational or unit-specific information. When in doubt, your unit armorer and the current published manual are the authority.