Evolution of the M4 Carbine

PEO Soldier, the Army acquisition agency, have published a timeline showing the 62 ECP (Engineering Change Proposals) that have been applied to the M4 Carbine over the past 18 years1. The original specifications called for 600 Mean Rounds Between Stoppage. The current reliability of the Carbine is over 3600 MRBS. COL Douglas Tamilio writes [PDF Link] …

The Army is committed to continuous improvement in all of our weapon systems. For example, the reliability requirement for the M4 is 600 Mean Rounds Between Stoppage (MRBS). The demonstrated current reliability is over 3600 MRBS as a result of our continuous improvement program. To date there have been 62 improvements to the M4, which include improvements to the trigger assembly, extractor spring, recoil buffer, barrel chamber, magazine and bolt. The M249 SAW, the Army’s Squad Automatic Weapon, has a reliability requirement of 1200 MRBS and yet today demonstrates a reliability of over 23,400 MRBS.

M4 Evolution Timeline. Click to here to expand.

Yesterday I wrote about the next set of changes to the M4 that the Army proposes to make, which include adding ambi control, a piston and heavy profile barrel. Clearly the Army does not believe the M4 has reached the end of its useful lifespan.

It appears that the M4 replacement testing is over before it began and that any testing that does take place will be to pacify the congressmen and women who are calling for the M4 to be replaced.

  1. The M4 entered service in 1997, but development of the XM4 carbine started at least as early as the mid-80s. 

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • Lance

    Good article one good reaon why the Army is staying with M-4s is that over 90% of the men who carry it like it. And most reports of the gun jamming are from men who misuse the carbine, like useing as a LMG which no rilfe it ment to do.

    I dont even think the Military will even need to test other weapons to appease Capital Hill. The main people who wanted to replace the M-4 ar either voted out of congress last year or in minority satatus and with the Obama adinistration in power I dont think the military will get much R&D funds ofr at least 4 years if not 8 (HOPE NOT HOPE NOT)!

    Another reason the M-4 will stay is that the other military services dont want to switch either. Both Navy and Marines are useing M-16A2s and M-16A4s and have stated they have no planse for the next decade to replace them. The Air Force is also keeping the carbine. The USCG is no longer in the Military and since DHS uses M-4s (FBI, CIA, NSA) in there armed units they wont switch either.

    In order to goto a new gun its logical to goto a new caliber. We would have to leave NATO for that. 6.5, 6.8, .243, and .280 calibers are great in testing. BUT since there not a NATO caliber they wont go into mass production.

    I think still the Colt Monolithic the H&K 416 and the Ruger SR-556 will be the main design canidates to be the M-4A2.

  • XC

    Hi – Love the chart but my old eyes can’t really decipher it. Can you post a higher resolution one?


  • Eric

    Clicking does not make it any bigger.

  • JGS

    Is there a link to a larger time-line picture? I can barely read it, even after clicking on it to expand…

  • please don’t use images which are too small to read in your blogs. that is all.

  • d

    sounds like they need to increase the MRBS requirement, 600 seems too low to begin with

  • Gutao

    IMNO (Im my noob opinion) a long stroke gas piston,and a maybe,a heavier caliber (6.8 SPC/6.5 Grendel) would make the M4 a beast,plus a folding stock,ant it would perfect IMNO.

  • SpudGun

    According to reliable sources (ie. the video games I play), I think we’re not to far off from a reliable and man portable laser / plasma / rail rifle.

    If ‘space guns’ are going to be available in the next 10-15 years, there’s no point in spending huge amounts on re-inventing the wheel.

    I know it sounds far fetched, but plasma and lasers are already being tested with aircraft weaponry.

  • Oops, sorry guys. I have updated the post with a link to the high-res verion of the chart.


  • Lance

    I dont think lazers will come soon. But there intrest in caseless amoo which is being experinmented on.

  • Big Daddy

    As a soldier who faces an enemy all I would want was a weapon that I could aim, shot and kill the person I was aiming at, period. We do not have that right now. We did before Vietnam but those weapons had their issues and we did need something better. If you study the history of weapons the Belgium company FN had it with their FN FAL and a .280 caliber British round, but we forced the 7.62mm on everybody and we were wrong.

    We have the technology to return the rifleman in an infantry squad the ability to do that again. Why do the idiots at the DOD not understand this?

  • jdun1911


    Ruger SR-556 piston produce a lot of wear in the upper and extension tube just like other AR15 piston via bolt tilt. I haven’t heard any thing about the HK416 but one can assume their bolt will also tilt.

  • Lance

    Except for full auto fire the 7.62 NATO round is better than .280. Better accracy and long range power. The preciced need for full auto is whats wrong.

  • Lance

    I agree but I say in a piston AR the SR-556 is the best of the 2. Ruger may work on the military model to keep wear down.

  • subby

    Its a shame noone offers or is willing to test these new advanced rifles in 3 simulated environments. sub zero temperatures, in the wet and high humidity and in the dusty deserts.

    Then finally people like FN can stop selling their hyperbole to the army and we can get a rifle that actually works. Where are the retired rich patriots when you need them?

  • Lance

    No the reason the M-4 is still here its because solders like it. If the guys like it then theres no need for looking at some FN peice of junk.

  • Big Daddy

    Lance were ever in the Military? Did you ever even fire an M-16 or AR-15.

    • Big, I know what Lance does for a job and that he is qualified to an opinion on the AR-15.

  • HK_USP_45

    OK, back to topic (I love how any mention of an AR gets into a pissing contest over which caliber is better, which rifle is better suited to military issue, etc. WTF does 7.62 vs .280 British (?????) have to do with the AR? For Frack’s sake, let it go):

    I’ve had a question on the topic, and maybe someone can answer it. Back in the 80s there was a M16 carbine, I believe called the CAR-16. Not even sure, but it seemed I saw pics of SOF using it in Vietnam, towards the end. 1. What ever happened to the CAR?
    2. Why is it not listed on this evolution diagram?
    3. How is it related to the M4
    4. What are the difference between the two?

  • Lance

    The CAR-15 was well liked in NAM. But due to congressional budget cuts in 1970 the military cut production off.

  • Destroyer

    a M4 upper with gas piston action, heavier barrel, and 6.5x39mm chambering is the answer…

  • zach

    I have to agree with Lance, weaponology (lol) and shows like that always say that there are reliability issues and that its too small of a round, like that one dumb “Top 10 Battle Rifle’s” that ranked the M14 10th and the AK-47 1st. I think the M16 platform came in 7th or something. But if the soldiers like it, then it’s obviously a good weapon. Although I believe the HK416 is a fantastic weapon, super reliable, accurate, and the controls are identical to the M4

  • HK_USP_45

    Zach, I agree. And I think most criticisms of the M4/M16 from the guys in the field are about the caliber, not the rifle itself. The AR rifle platform is outstanding.

    lance, aren’t a lot of M4 uppers made by FN for Colt to supply for the gov? I would hardly call FN junk. The Scar might need some work, but as I said, I wouldn’t say FN makes shoddy products. I think with the Scar, FN was trying to get it fielded quickly, and it’s got some stuff to work, but it will get there. No matter how much you beat up and test a combat weapon, you can’t simulate the battlefield. Stuff that didn’t break in reliability testing, will break in the hands of a soldier or Marine. Let’s face it, we find new and better ways at breaking things, and we’re good at it. I for one can tell you that with the level of creativity a Marine can come up with to break things, I think it’s an artform. But if the military uses the Scar, it will evolve into an outstanding product, just like everything else FN offers. That’s what good companies do, they take feedback from the field to make design improvements.

    Very few battle weapons have been perfect right out of the gate. I mean, look how long it took to get the M16 right, and look at what it’s become now. You can say that about just about every weapon we’ve fielded. That’s why everything has an “A” followed by a number after the nomenclature.