How they’re improving the M4

Generic M4 as issued in 2013. To be converted to M4A1.

Generic M4 as issued in 2012. To be converted to M4A1 by 2014. (PEO Soldier).

The Army intended to have a two-pronged approach to infantry weapons: a new Individual Carbine, and continuing improvement to the M4, which was “good enough” and expected to soldier on for many years — for one thing, a new carbine would have to be built, and the Army already has more than a half million serviceable M4 and M4A1 carbines. But as we’ve seen, the IC design bake-off cratered when none of the experimental new guns matched (let alone beat) the reliability of the M4 controls. Much like a medical trial is canceled if the experimental group does markedly worse than the controls taking a placebo, the Army called off the IC competition.

Of course, the subtext to the decision is the command climate in Washington, where the SecDef’s view of the Army is something to slash and bleed of a “peace dividend” for higher-priority domestic redistribution. But even the Pentagon’s crack flacks, whose mission brief is to make Capitol Hill and the press believe five impossible things before breakfast, would have a hard time selling Congressional and media skeptics on a new rifle that wasn’t as reliable as the old one.

So. the IC competitors are now, in most cases, back at the drafting station trying to wring more reliability out of their designs (some of which are pretty mature designs selling internationally, some of which borrowed their maturity by being largely M4s anyway, and some of which are both).

Without decimal places, modern US weapons are 100% reliable. Running out  the decimal places, the M4 is 99.983% reliable (the 3 repeats infinitely) in terms of MRBS (1/6000). (The SAW is even better, with “four-nines” reliability: 99.9957% (1/23,400 on stoppages). Any engineer will tell you that those numbers are hard to beat, and in fact, they’re murder to improve. It is much easier to get the gun from one stoppage in 30 rounds to one in 300. It is more difficult to take that from 300 to 3000. To take it to one stoppage in 6000 required decades of work by hundreds of engineers in government and industry, and hundreds of (mostly) minute tweaks to rifle, magazine, and ammo. To get from 6000 to 6100 is going to be tough; by now the graph of effort to result is proceeding nearly vertically.

The rifle is a resonant system, and changing any one component may influence any others. And when your system that reliable to begin with, any random change is more likely to lead to more unreliability. This leads to what academic Charles Perrow has defined as “normal accidents” in the book of the same title. Perrow noted that as systems got more complex, interventions meant to eliminate observed failure modes and increase reliability and safety are increasingly likely to introduce new and unintended failure modes. He points out these unintended consequences in, for instance, the Three Mile Island nuclear disaster. The same mechanisms were at work in the American Airlines Flight 587 disaster in New York City in 2001. Because several accidents have involved inflight upsets, American arranged for its pilots to undergo upset-recovery training in aerobatic airplanes. The pilot flying 587 applied the control forces he learned in the nearly indestructible Extra aerobatic airplane, in a relatively fragile Airbus, with fatal consequences. Nobody saw it coming — but Perrow saw that coming.

M4 Carbine Improvement Timeline. Click to embiggen.

M4 Carbine Improvement Timeline. Click to embiggen.

Despite the difficulty and the ever-present threat of unintended consequences, several interdependent teams of experts continue to work to massage the M4. The manufacturers’ engineers, the Naval Surface Weapons Systems facility in Crane, Indiana (which works, among other things, joint SOF weapons), and the Army’s Ordnance experts and Program Executive Office for Soldier Systems (PEO Soldier) all continue to pursue the combat asymptote, the perfectly reliable weapon.

So what non-random changes are possible, and what are in the pipeline for the individual weapon? Of these various sources of improvements, PEO Soldier is the most forthcoming about its plans. Crane is imbued with SOF reticence, and manufacturers seek to keep proprietary improvements away from their competitors. So let’s look at PEO Soldier’s plans for the M4 series:

The first thing the Army is doing is long overdue: it is dumping the awful Colt-designed (and Army-demanded) three-round-burst mechanism. (The only reason the burst mechanism was specified was to avoid spending money teaching combat soldiers to fire accurately on automatic). The burst mechanism didn’t reset to zero, meaning you could be set on burst and get one, two, or three shots per pull, depending on what your last burst consisted of. And the trigger pull is different depending on where the burst mechanism is in its cycle. The elimination of burst is part of an overall conversion of all fielded M4s to the formerly SOF-specific M4A1. Along with full-auto capability instead of the crappy burst, the principal benefit is a thicker barrel that is slower to overheat (and, unfortunately, slower to cool off. In engineering, there is no such thing as a free lunch). PEO Soldier:

There are several benefits to upgrading M4s to M4A1s. Compared to the M4, the M4A1 has full auto capability, a consistent trigger pull, and a slightly heavier barrel. The heavier barrel is more durable and has greater capacity to maintain accuracy and zero while withstanding the heat produced by high volumes of fire. New and upgraded M4A1s will also receive ambidextrous fire control.

The ambi fire control will be welcomed by southpaws, and will be useful in some urban cover as well. But the best news in Phase I is the banishment of burst.

Along with the conversion of the extant M4s to A1s, the Army is setting up a contract for new M4A1s, with 24,000 in the initial order. The contract is set up so that the other services can also order carbines against it — A1s or the original M4s if they prefer a crappy trigger. The conversion process is going to take place Army-wide, starting in July or August.

Phase II explored two possible areas of improvement — Bolt Carrier Groups and Rails Systems. The Rails component has not had a winner announced yet, but the BCG competition, with 11 firms competing for the prize, ended much as the Individual Carbine competition did, and for much the same reasons: none of the new contenders could beat the current champ:

PM SW completed its best value M4 bolt and bolt carrier assembly competition in April 2012, though the competition was scheduled to conclude in summer 2013. More than six months of testing and evaluation determined that none of the 11 competing designs met the overall requirements outlined in the solicitation. The M4’s current bolt and bolt carrier assembly outperforms the competing designs in the areas of reliability, durability, and high-temp/low-temp tests. The Army saved nearly $2 million as a result of the early completion of the competition.

Not surprising. The only change we’d make to the present M4 BCG, if we were kings, is largely cosmetic: we’d return to a satin chrome finish. That makes it stupid easy to clean the bolt and carrier, an important thing on a gas impingement gun that uses its bolt carrier as a cylinder and bolt as a piston.

Phase III, according to a briefing PEO Soldier presented(.pdf) in 2011 (which has an earlier version of the M4 Continuous Improvement slide shown above), is a further evaluation of the M4A1 operating system.

In other words, they’re institutionally unhappy with a weapon that’s very, very good. Other improvements continue, but PEO Soldier is in a bit of a jam: past improvements to the M4 system have been so successful that almost any further attempt at improvement degrades some aspect of the weapon’s performance. It’s not an entirely bad jam to be in: “Help, our gun’s too reliable!”


The system tells us that this is the 1,400th post on, which launched January 1, 2012. We hope some of them have been of interest and of use to you. Expect more of the same. –your Eds.

5 thoughts on “How they’re improving the M4

  1. McThag

    I wonder if the new (old) trigger will find its way to the Marine’s M16A4.

    It weren’t just the Army demanding that 3-round burst way back when.

  2. Daniel E. Watters

    I really wish that Dan Shea would reinterview Dave Lutz, now that he has retired from Knight’s Armament. Shea had previously interviewed Lutz for the now defunct “Machine Gun News” right around the time that Lutz had retired from the US Marine Corps. During his career in the USMC, Lutz had his fingers on several major small arms programs: the development of the M16A2 rifle, the Corps’ ill-fated attempt to adopt the XM4 carbine during the 1980s, and the acquisition of “surplus” US Army M240 co-ax for conversion into M240G. The MGN interview only concentrated on the first of these, and Lutz gave his reasoning behind nearly every PIP modification. The 3rd burst was a political bargain, as there were certain individuals who wanted to make the PIP rifle semi-auto only. Lutz admitted that if he had only known of its existence, he would have liked to have tried the 6-round burst cam shown in “The Black Rifle.”

    1. Hognose Post author

      If he was involved in the 240 program, he’s a great saver of American lives. The MAG was and is so much better than the 60 it’s shameful.

  3. Aesop

    USMC took three-round burst for political reasons, to get the rest of the much-needed A2 improvements.
    If someone involved with that horsetrade also got the .mil to finally scrap the Pig for the FN MAG, he deserves a bronze statue at the front of SOI or in front of the 0331 schoolrooms.

    Anyone who adopts a weapon that can be reassembled corrctly with parts in backwards ought to be horsewhipped.
    Hanging on to such weapons until the point that pieces regularly fall off of them on patrol should be grounds for a trial for treason, followed by a fine public beheading.

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