How to Field Strip, Disassemble, Reassemble, Clean & Maintain a Military-Issued M4 Carbine Rifle
At home, owning a rifle is either for sport or fanaticism. Having an M4 Carbine, however, is illegal. Only government agencies can own and use the Colt M4 Carbine, most notably— the U.S. Military. And for a soldier, that rifle means life or death, so it's the number one concern for soldiers serving in Iraq or Afghanistan today. For a soldier, cleaning and maintaining their issued weapon is always priority, because an unclean rifle can resort in jams, misfires, and other malfunctions.
From basic training to the desert, soldiers are taught that their weapon is their only means of survival, and it will remain by your side even after your fellow soldiers are dead and gone. In basic, they teach you the components and how to disassemble and reassemble your rifle. Then, they show you the fundamentals to cleaning, but it isn't until you graduate and arrive at your first unit that you really understand how to clean your weapon. You learn the finer points to rifle cleaning from your command and teammates. They show you their tips and tricks from past personal experience, and they show you how to detail it for inspections.
Yet, you can't fully comprehend the importance of cleaning your weapon until you're in a war zone. In the states, you only clean it when your command demands it. Besides that, you hardly ever see your issued rifle, and for the most part, you're glad because you think it's just a mindless chore they have you do to pass the day, a sort of punishment. On tour in Iraq or Afghanistan though, that's a whole 'nother story.
In a combat zone, you hold your trusty M4 rifle everyday, all the time— you walk with it, eat with it, sleep with it, shit with it— fight with it. It's your life force, and no one else is responsible for its well-being except you. You can't depend on somebody else to save your ass in a firefight when your rifle jams because you were too tired to clean it that morning. They're too busy saving their own asses. And that's when you really find out it was never a chore. It was never punishment. It was preparedness. You wouldn't want to be known as the guy who died for being a lazy dumb-ass, right? Who would want to be that guy?
But what if you encounter situations like these overseas?——
Okay, natural and manmade sandstorms don't happen everyday, but as much as you know about cleaning, you could never be fully prepared for the sandy environment in the deserts of the Middle East. Sand is your rifle's worst enemy— it gets everywhere. It finds every nook and cranny and jams itself in there. It finds ever little spring, every little notch, and embeds itself.
A 25th Infantry Division soldier once wrote, "The M4 Weapon in the deserts of Iraq and Afghanistan was quick to malfunction when a little sand got in the weapon. Trying to keep it clean and sand-free was impossible while on patrols or firefights. Sometimes we spend more time cleaning the weapon than firing it."
But nonetheless, whenever you find the time to perform a full disassembly and cleaning, you need to do it to the max, to get rid of every little grain of sand. Eventually, cleaning your weapon will get to the point of relaxation. It will calm you— help keep you peaceful— keeping you busy and forgetting where you are. Sooner or later, you'll start to enjoy it— to crave it.
Now these aren't expert instructions, but an overview and basics of disassembling, cleaning and maintaining the M4 carbine rifle. And this doesn't apply to only soldiers, it applies to anyone who owns an M4, which in the civilian market should be slim to none, since M4's are illegal to own personally because they are considered to be machineguns. This is because of the law stated in Chapter 44 (Firearms), Title 18 (Crimes and Criminal Procedure), Section 922 (Unlawful Acts) of the United States Code. It basically states that a machinegun is legal only if it was manufactured and registered with the ATF (or BATFE) prior to May 19, 1986. But I say the military should let soldiers keep their weapons as parting gifts, but I guess that's a pipe dream.
Still, if you don't have an M4 rifle, these instructions also work quite well with M16 rifles, or more commonly— AR-15 model rifles. The AR15 rifle is the semi-automatic civilian version of the M4. Its barrel is longer, and it doesn't have the notch for a grenadier attachment, nor the three-round burst option.
When you're in the field or on a mission, your M4 rifle may need some basic maintenance performed. In Iraq or Afghanistan, it will always needs a little care, but I wouldn't suggest doing a takedown during a sandstorm. Only perform a field strip if under cover or in calm winds while in a desert environment, to prevent unintentionally getting your weapon dirtier than cleaner.
The steps for field-stripping are easy. All you are doing is giving it a quick dust off and lube, which means there isn't much to disassemble.
Step 1 Clear the Weapon
- First off, make sure that your firearm is on safe by checking that the safety selector on the left side of the lower receiver is set in the SAFE position. It should always be on safe unless you are performing a functions check or firing it. If the weapon has been fired already (not cocked), you will not be able to engage the safety. Continue on with these steps to see how to engage the safety.
- If necessary, remove the magazine by pressing the magazine release button on the right side of the lower receiver. Set the magazine away from arms reach, especially if loaded, to prevent any unintended accidental loading.
- Pull the charging handle rearward and lock the bolt in place by pressing on the bottom of the bolt catch on the left side of the lower receiver. Ease up on the charging handle while pressing the bolt catch to allow the bolt to move forward until it engages the bolt catch.
- Return the charging handle to its full forward position and place the selector lever on SAFE.
- Visually inspect the upper receiver and chamber for any ammunition. Do not physically inspect (which means DO NOT stick your finger inside the upper receiver), because the bolt may slide forward and catch your finger. Remove any ammo if necessary.
- Press the upper portion of the bolt catch (the bolt release) to allow the bolt to slide forward.
- Now place the selector lever to SEMI and squeeze the trigger. This is basically just a double check to make sure there is no ammo in the chamber, to prevent injury when disassembling.
- Pull the charging handle rearward fully and then release to allow the bolt to return to the full forward position.
- Place the selector lever on SAFE and close the ejection port cover on the right side of the upper receiver. Always close the ejection port cover unless firing, to prevent damage and keep the bolt clean.
If you try to open the receiver with the bolt locked to the rear, it could seriously injure you or someone else. Make sure you follow the full clearing procedure properly.
Step 2 Pop Out Rear Takedown Pin
Now that your weapon is considered safe to disassemble, pop out the rear takedown pin on the lower receiver all of the way. Do not remove the second takedown pin (commonly called the pivot pin) near the barrel, because this pin will allow the upper receiver and barrel to swing down for access to the bolt carrier group, without fully disassembling.
I always found that it was easy to remove the takedown pin by grabbing the lips of the pin on the right side with my fingernails and simply pulling it out. Sometimes it can be a little stubborn, so you may need to pop it out from the left side with something. Sometimes your finger will be enough to get it started, but sometimes you may need to use your Gerber or barrel rod from your weapons cleaning kit. You can also use practically anything else, like a pen or pencil.
Once the pin is out, swing the receivers apart for inner access.
Step 3 Pull the Charging Handle
Pull the charging handle rearward on the upper receiver. You will not be able to remove this however, until after you have removed the bolt carrier group.
Step 4 Remove the Bolt Carrier Assembly
Pull the bolt carrier group out of the upper receiver and set down close by you on a clean surface.
Step 5 Remove the Charging Handle
After the bolt carrier group is removed, the charging handle will slide out easily. Remove and set down on the clean surface by the bolt carrier group.
In the field or when off base in hostile territory, you never want to disassemble your weapon more than this. Do not disassemble the bolt carrier group, because you could possibly misplace a piece, which could be detrimental in an unexpected attack. Plus the whole idea of a field strip is to only give it a basic dust over, to remove any large obstructions or coarse dirt and sand from hindering your M4, causing jams or double feeds.
Step 6 Clean
Now it's time to clean and inspect your weapon. Inspect the bolt carrier group and charging handle for any damage. Proceed to dust off the two components with a rag or barber brush.
Next, use the rag or barber brush to clean the inside of the upper and lower receiver. Also, quickly wipe down any other noticeably dirty areas on the outside of the firearm if you have time. If you want, you can even remover the buffer assembly and action spring from the lower receiver and gently wipe them off. To remove, simply use your fingernail to press on the release button by the buffer assembly.
After wiping your weapon down, you may apply a light coat of CLP (cleaner, lubricant, and preservative), which I usually just refer to as oil or lubricating oil. In the desert environment, it's a good idea to apply a very light amount of oil to your bolt carrier group, because sand and dust will cling to it in seconds, defeating the purpose of cleaning in the first place. Do not lubricate the outside of your weapon in Iraq!
Step 7 Reassemble
After a light cleaning is performed, insert the charging handle back into the upper receiver, but do not push forward all of the way. Only slide it in through the groove until it is locked from falling out, which will leave most of the handle hanging outward.
Install the bolt carrier group into the upper onto the charging handle. Now, the bolt carrier group will not install if the bolt assembly is not pulled out— pull out the bolt assembly, then install. Once in place, push the bolt and charging handle forward fully until they lock in place. Make sure to close the ejection port cover. If the port ejection cover was already closed, pushing the bolt forward will open it, so you'll have to close it again. If necessary, reinstall the buffer assembly and action spring.
Now, just bring the upper and lower receiver together and push the rear takedown pin back in place and you're done— almost.
Step 8 Functions Check
To make sure your weapon is still capable of firing, you'll need to perform a functions check on the M4. It is very important to perform a functions check any time you disassemble you rifle, no matter how small the disassembly was.
A. Check "SAFE"
Place the safety selector lever on the left side of the lower receiver on SAFE. If it will not go on safe, pull the charging handle back fully and then let go to release the bolt forward. Place on SAFE. Pull the trigger to the rear to test the safety. The hammer should not fall, which means the trigger will not depress all of the way, and you will hear no click.
B. Check "SEMI"
Now, place the safety selector on SEMI. Pull the trigger to the rear again, holding in place. The hammer should fall, which means the trigger will depress all of the way and you will hear a click.
While holding the trigger to the rear, pull the charging handle to the rear fully and release the bolt back to the forward position. Release the trigger and pull it to the rear again. The hammer should fall, which means depressing all of the way and clicking.
C. Check "BURST"
Place the safety lever on BURST. Pull the charging handle again and release. Pull the trigger to the rear again, holding in place. The hammer should fall. While holding the trigger in place, pull the charging handle to the rear three times and release. Release the trigger and pull it back to the rear again. The hammer should fall. Click. Your functions check is now complete.
- If at any time during the functions check an unexpected result happens, disassemble the weapon again to ensure for proper assembly, or to check for previously unnoticed damage.
During a little down time, it's a good idea to do a complete disassembly of your rifle. The full disassembly is required to perform a full cleaning, whereas the field strip is for immediate cleaning in the field. There are different approaches to disassembling the Colt M4, so it's not necessary to follow these steps in the presented order, nor is it required to complete every step. It's up to you and your allotted timeframe, but I would get as detailed as I could.
Step 9 Clear the Weapon
First, always perform the clearing procedure before doing any maintenance, as detailed above in the field strip section. Do not skip this step!
Step 10 Remove Sling
Before you get all crazy with your stripping, the easiest thing to take off is usually the best thing to start with. Remove your sling from the buttstock and the sling swivel under the front sight assembly.
Step 11 Remove Handguards
If you plan to take the handguards off, now's the time. Simply press down the handguard release buttons and slide them up the rail of the RAS (Rail Adapter System) handguard assemblies and out near the front sight post. Remove all handguards. The handguards are interchangeable because they are identical, so don't worry about which one went where.
Some of you might not have a RAS, but just the standard handguard assembly. If so, skip to the next step.
Step 12 Remove RAS Handguard Assemblies
In order to remove the RAS handguard assemblies, place the bottom of the buttstock against a firm surface (I usually just use my leg). Pull down on the slip ring until the lower lip of the lower handguard assembly is clear. Pull out and down on the lower handguard rail assembly until the upper lip is clear of the hand guard cap by the front sight post.
To get the upper RAS handguard assembly off, you will need a hex key to loosen the bolt near the bottom of the assembly. Once loose, simply repeat the instructions for removing the lower handguard assembly.
If you don't have the RAS assemblies, but the standard handguard assemblies, then it's pretty much the same, minus the hex bolt.
Step 13 Remove Buttstock Assembly
Before you can actually remove the buttstock assembly, you need to fully extend the buttstock assembly. When fully extended, grasp the lock release lever in the area of the retaining nut. Pull the lever downward to eject the pin from the lower receiver extension. While holding the lever out, slide the buttstock to the rear to separate the buttstock assembly from the lower receiver extension.
Step 14 Separate Upper and Lower Receivers
Now comes the time to separate the upper and lower receiver. It's exactly like performing a field strip, but now you're fully separating them. This time, pop out both takedown pins, the rear and the pivot, then detach the upper and lower receivers.
Step 15 Remove Buffer Assembly and Action Spring
To remove the buffer assembly (which looks like a large thick bolt) and action spring, simply depress the buffer assembly in a small ways. Depress the buffer retainer with your thumb tip or with something solid, then release the buffer assembly until it passes the retainer. Now just remove the buffer assembly and action spring fully.
To separate the buffer assembly and action spring, it's as simple as pulling them apart.
Step 16 Remove Charging Handle and Bolt Carrier Assembly
As outlined in the field strip section, pull the charging handle rearward on the upper receiver, moving the bolt carrier group with it. Pull the bolt carrier group out of the receiver, and then fully remove the charging handle from the receiver.
Step 17 Disassemble Bolt Carrier Assembly
Now time for the fun part— the bolt group. Set aside a clear area for the disassembly of the parts, because you don't want to lose any of them. If you lose a part, your weapon will be rendered useless, not to mention your leadership will have your ass, so be careful to keep track of the pieces during disassembly and cleaning.
A. Remove Retaining Pin
To remove the retaining pin on the bolt carrier group, simple grab the triangle-shaped head of the retaining pin with your thumb and index fingernails. Pull it out fully. You may need to turn it back and forth until it slides out. Make sure you do not spread or close the legs of the retaining pin though.
B. Remove Firing Pin
Once the retaining pin is removed and set aside, simple tilt the bolt carrier until the firing pin falls out into your hand. If it does not slide out easily, give it a little whack and it will come right out.
C. Push Bolt Into Carrier
With the firing pin set aside, push the bolt assembly fully into the bolt carrier, into the locked position.
D. Remove Cam Pin
With the bolt pushed into the carrier, find the square-headed bolt cam pin— it holds the bolt into the carrier. Turn the cam pin one-quarter turn and lift out of the bolt and carrier. Set aside.
E. Remove Bolt Assembly
With the bolt camp pin out, you can pull the actual bolt assembly from the key and carrier. There's a certain way it goes in, so make sure you remember how it comes out. Even if you don't remember, you'll still figure it out later on, because you can only install the cam pin back in through one side of the bolt. Set the bolt carrier aside.
F. Remove Extractor Pins
Take the bolt in your hand and find the extractor pin. Remove the extractor pin by pushing on it with the firing pin. It should come out pretty easy, but be careful not to lose it. It's one of the smallest parts and the easiest to misplace. Set aside.
G. Remove Extractor
Now that the extractor pin is out, lift up gently on the extractor with spring. Take extra care as to not separate the spring from the extractor, even when cleaning. Sometimes you can just put it back in if it accidentally comes apart, but it's probably best to just go to your company armor.
The above steps, 9-17, are the basic procedure for disassembling the Colt M4 carbine rifle for general maintenance. Like I said before, you don't have to perform them in the presented order. You could clear the weapon, then go straight to separating the upper and lower receivers, to removing the charging handle and carrier bolt assembly, to disassembling the carrier bolt assembly. Then you can do the more minor items, like the handguards and sling and buttstock.
When overseas, if you know you have a good amount of time to clean your weapon, then the order doesn't really matter. But there's always a chance you could get called out on a mission, so cleaning the main components (bolt carrier group, buffer assembly, etc.) first might be a good idea, before you disassemble anything else.
Step 18 Optional Disassembly
If you want to go further with your cleaning, the following items could also be removed for cleaning, without any needed armor tools.
A. Carrying Handle Assembly
The carrying handle assembly should only be removed if damaged because you could lose your zero, which means when it comes down to a firefight, you probably won't be hitting anything. You should also take care when cleaning the carrying handle assembly, because you can also ruin your zero. Chances are you don't even have a carrying handle on your weapon, because you probably replaced it with an M68 CCO.
Before removing, you should first observe the position of the clamp on the rail in relation to the carrying handle, so you remember exactly where it is for reassembly.
Remove by loosening the round carrying handle assembly nuts. It usually takes about four to five complete rotations to loosen them to the desired amount. Loosen the clamping bar from the left side of the upper receiver's rail and proceed to lift off the carrying handle.
B. Magazine Catch and Button
Removing the magazine button and magazine catch is a simple step to perform. It's quick and easy, so I always removed them for a quick cleaning.
To remove, push the magazine button on the right side of the lower receiver in as far as it will go using a pen or similar item. On the left side of the receiver, the magazine catch will pop out. While holding the button in, turn the mag catch counterclockwise to unscrew and remove it.
With the catch off, gently release the magazine button and remove, along with the magazine catch spring. Take care as to not lose any of these parts. With small parts, sometimes it's best to clean immediately after removing, for prompt reassembly to prevent loss.
C. Port Ejection Cover
This is something that you really shouldn't take off unless you are completely anal (like me). It really shouldn't be taken off unless damaged and in need of repair. If you do decide to take it off, you could get yelled at from your leadership, or you could simply have more to clean. Most people won't do this— I only did it a few times.
Remove the retaining ring near the handguard assemblies slip ring. Slide the headless grooved pin out to the rear. Sometimes the pin can get stuck on the forward assist housing, so a little force will be needed to remove it. When removing the headless grooved pin, be ready to catch the cover spring and ejection port cover. If you don't, you may accidentally lose them, mostly the small spring.
D. Sling Swivel Mount
Every once in a while, I found myself removing the sling swivel mount from the barrel by the front sight post. Do not remove the actual sling swivel from the mount, because you will not be able to replace the tubular rivet back, and you will have to have your armor repair it. Only take the mount off the barrel.
There are two spring pins to remove on the mount. Knock them out with an awl (which you probably don't have) or something similar. Be creative— sometimes I used the firing pin to simply push them out a bit, then I used my Gerber to grab them and yank them out. Do not pound on your firing pin though! You're better off using a pen or pencil to push them out until you can grab them with your Gerber.
Next, lift the swivel locking bar up and out of the swivel mount, then remove the swivel mount from the barrel.
Step 19 Additional Things to Consider
If you are in Iraq or Afghanistan, you probably have more than just the M4 itself. You could be a grenadier, which means you would have to take off the M203 and perhaps a leaf sight from the weapon. There's also the backup iron sight that is installed when the M68 CCO is mounted. DO NOT attempt to remove the backup iron sights! You could also have a AN/PEQ-2 infrared illuminator for nighttime missions, which means you would also have the PEQ mount. There are other additions to your M4 carbine that could be attached to the rail system, but only remove and clean those items that you know can come off and be cleaned. Removing any sights that have been zeroed will require you to re- zero them, so DO NOT remove them.
Every once in a while, it's a good idea to disassemble your 30-round magazine for cleaning. Lots of sand can find it's way in there, causing the spring to malfunction. Neglecting your magazine is just as bas as neglecting your firearm. Before disassembling your magazine, make sure there are NO rounds loaded.
Step 20 Base Plate
Remove the base plate on the bottom of the magazine. You'll need to use a screwdriver or Gerber to grab the center notch and push to slide the base plate off.
Step 21 Catch Spring and Follower
With the base plate off, grab the magazine catch spring and pull it out. The follower is attached to the catch spring, so it will come out also. When out, simply separate the two components.
Cleaning your rifle is the most important thing you can do to save your life while on tour in Iraq, or any other hostile environment. If there isn't an adequate amount of lubrication on your bolt carrier group, it can lead to misfires and jams.
Step 22 Cleaning Materials
Aside from the standard military-issued olive drab weapons cleaning kits that soldiers get, there are a few things you'll want to have extra.
Almost everybody I knew (and me) had a barber brush in their cleaning kit, which is the best item to have for cleaning and dusting off. It's probably the most valuable asset to your kit, because it can easily get sand and loose debris off quickly, and it's great for a quick brush off after field stripping.
Other useful items for your cleaning kit include clean rags, pipe cleaners, Q-Tips, dental picks, cleaner lubricant and preservative (CLP), cleaning swabs, and the items included in your issued kit, like the toothbrush, chamber brush, cleaning rod, and bore brush. If you have a M203, you'll need to also have a large bore brush for that.
Step 23 The Cleaning Process
A. Upper Receiver
For the bore of your barrel, use some small cleaning swabs, CLP, and the bore brush to get the carbon residue out. First, use the bore brush with the cleaning rods, and pull the brush down the bore a few times in the direction of bullet travel.
For the rest of the upper receiver, inside and out, use rags, a cleaning brush, and CLP to remove powder fouling, corrosion, dirt, carbon, and rust.
Clean the bolt lugs with some CLP and the chamber brush. Use CLP and the bore brush to clean the barrel locking lugs and gas tube. Also, using pipe cleaners inside of the gas tube will help, but make sure not to leave and small pipe cleaner fibers in the tube that could cause blockage. Every once in a while, I use a small Q-Tip in there, but again, do not leave behind anything that could cause blockage.
For the star chamber, use the bore brush and CLP, but it's really difficult to actually see if you are getting all of the carbon, so do the best you can. Q-Tips also help get the hard to reach areas, and sometimes a dental pick will help loosen stubborn carbon from the chamber.
B. Lower Receiver
For inside and out of the lower receiver group, use rags, a cleaning brush, and CLP to remove powder fouling, corrosion, and dirt. Q-Tips are also very helpful for the trigger mechanism and hard to reach spots.
Clean the buffer tube and buttstock tube with a rag attached to a holder. Most of the time I just push a large rag down the tube and twist it around a few times, then yank it out. Do this a few times to get all of the sand and other material out.
Clean the buffer action spring with some CLP and a rag. Also, use the barber brush to apply a very light coat of oil to the spring, then wipe off with a rag.
C. Bolt Carrier Group
Clean the outer and inner surfaces of the bolt carrier with rags and CLP. Q-Tips are great for getting to the hard to see and reach areas.
Use a worn bore brush to clean the carrier key, so as not to damage it. Clean the firing pin recess and firing pin with CLP and a brush. Q-Tips, again, are a valuable tool for cleaning the carbon from the recess.
Clean the firing pin hole with a pipe cleaner and CLP. Clean the carbon deposits and dirt from the locking lugs with CLP and a cleaning brush. Clean the areas behind the bolt ring and under the lip of the extractor with a brush and CLP. Using a Q-Tip for inside the extractor spring is useful, just be careful not to accidentally separate it from the extractor.
To clean the magazine parts, use a rag soaked in CLP. Make sure you dry all parts after lubing. Too much lubricant inside a magazine will collect tons and tons of sand, causing your spring to malfunction and cause misfeeds or no feeds at all.
For lubricating, this is the general process for all environments. Just beware that in Iraq, the desert sands make it hard to use too much CLP. The sand and dust will stick to your weapon in seconds if using too much CLP, so it's usually a good idea to keep the lubrication on the outside of your weapon down to a minimum. But make sure you DO lube your weapon!
"Unfortunately, an incorrect thought process persists among many officers and senior enlisted – that the rifles should be scrubbed clean as often as possible, and that oil should not be added to the weapon, for it will "attract dust and dirt". Soldiers and Marines have been dying because of this absolute garbage since the introduction of the Garand in combat operations in the Pacific during WWII. Proper lubrication is vital for any semiautomatic or automatic weapon." — Vuurwapen Blog
E1. Upper Receiver
Lubricate the upper receiver lightly, lubing the inside, bore, chamber, locking lugs, outer surfaces of barrel and front sight, and mating surface for carrying handle. Again, be careful of using too much in a desert environment.
Apply a few drops of lubricant to the threaded studs on the carrying handle. Lightly lube the clamping bar and both round nuts and mating surface. For Iraq, "light" means virtually none, especially for the nuts and studs.
For the front sight detent, depress and apply a few drops of CLP, then depress several times to work the lubricant into the spring, but less is better for the desert.
For the adjustable rear sights, use a slight amount of CLP to the elevation screw shaft, elevation knob, windage knob, and windage screw. Rotate the parts to ensure a good spread of lube, but in Iraq and Afghanistan, again... do this minimally.
For the bolt carrier group, lightly lube the charging handle and the inner and outer surfaces of the bolt carrier. Place one drop of CLP in the carrier key and apply a light coat on the firing pin and firing pin recess in the bolt. Lubricate the outside of the bolt body, bolt rings, and the cam pin area. Also, apply a light coat to the extractor and pin.
E2. Lower Receiver
Lightly lubricate the inner and outer surface of the lower receiver. Lube the lower receiver extension, buffer, action spring, buttstock assembly, buttstock lock-release lever, and retaining pin.
Generously lube the takedown pin, pivot pin, detents, and all of the other moving parts and their pins. In Iraq, "generously" means lightly. Make sure to get the trigger mechanism and springs, and get inside the hard to reach forward assist housing.
Now, I keep saying that in Iraq or Afghanistan, less oil is better, but to make this work for you, you need to clean your weapon on a general basis, which means every day or several times a day. To prevent any sand or dirt from unnecessarily inhabiting your rifle, when on missions, it's a good idea to bring along your kit and keep your barber brush in your pocket for quick dust-offs.
Use a muzzle cap when not using your rifle, to prevent dust and sand from entering your barrel. Sometimes I even left it on for missions. Most of the time, after you fire a round it will pop off, but sometimes it will stay on and melt to the barrel, so it is not recommended.
When traveling, inside a truck or in the back of a truck in the open air, carry a rag with you to cover the lower and upper receiver, to keep sand and dust out of the bolt carrier group.
Finally... clean, clean, clean. Being consistent with your maintenance is the best thing you can do to keep your M4 carbine rifle functional. That also goes for your M16 or AR-15.
To reassemble your M4, it's basically the reverse process of disassembling. I won't go in to detail here, because reassembling is pretty intuitive. Just do what you do when disassembling, just the opposite. There is no real order to assembling, unless you are in basic training or trying to pass the board, but I won't go into that, because I'm talking about the basics when in a combat zone situation.
Well, that's it. Through time, you'll find your own process of field stripping, disassembling, cleaning and maintaining your M4 carbine rifle, M16, or AR-15. You'll find out what works for you and what doesn't, and you'll realize what parts are the most important to clean with limited time before missions. This article just describes the way that works for me, but even I tend to change it up every once in a while. Just make sure you're having fun (which can be hard for some soldiers).