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Forget About the Army’s Old M4 Carbine Rifle: What An M5 Should Look Like

M4 Carbine Replacement
JOINT BASE ELMENDORF-RICHARDSON, Alaska -- Cpl. Johnny Hurst, assigned to A Company 3rd Battalion (Airborne) 509th Infantry Regiment, a native of Chicago, Ill., fires his M-4 carbine during a live-fire and movement-to-contact operation on the Infantry Squad Battle Course at Joint Base Elmendorf-Richardson, Friday, May 31, 2013. The Soldiers focused on core infantry skills such as fire team movement, communication, shifting fire, and once on the objective identifying and eliminating weapons caches and treating and evacuating casualties. (U.S. Air Force photo/Justin Connaher)

The U.S. Army is looking for a new weapon for infantry troops. After a half century of using the M16/M4 carbine series of weapons, the Army is looking for a weapon with increased range and lethality to deal with future threats. Although the search is still in its early stages, looking at the threats and the current state of small arms technology we can get an idea of what the service might shoot for–figuratively speaking–in a new soldier weapon.

The current U.S. Army weapon that the service eventually seeks to replace is the M4 carbine. An evolution of the original M16 introduced in the mid Sixties, the M4 is more compact and more reliable than its ancestor. Thanks to improved ammunition the M4 is able to penetrate lightweight body armor, and the use of optics allows soldiers to place rounds on target more accurately at longer ranges than ever before.

All of that having been said, the M4’s basic design precludes important upgrades. The use of a direct impingement operating system, although simple and effective, requires more frequent cleaning and alternatives exist. The 5.56-millimeter round, although lightweight and logistically appealing, is reaching a dead end performance-wise and a good argument exists that it needs replacing.

Now, let’s look at the threat and operating environment. Unlike many ground forces, the U.S. Army must be ready to fight literally anywhere and against anyone on the planet. Soldiers must be capable of engaging enemy troops at short ranges in cities while at the same time being capable of long range fire against enemies in open terrain. The weapon must be capable of controlled fully automatic fire in close quarters battle while at the same time having the energy to engage targets at long ranges. The cartridge the weapon fires must be capable of penetrating modern, advanced body armor while still tumbling shortly after entering the human body, creating lethal wound cavities.

Fulfilling all of these requirements, some of which are inherently contradictory is no easy task, and it’s easy to see why a conservative organization like the U.S. Army simply decided to upgrade the existing basic design over and over again. It’s difficult enough to meet this challenge, let alone demand a weapon with ten times the performance of the existing M4. Instead, we’ll envision a weapon that is a cost-effective improvement over the M4 with the capability to further grow down the road.

M4 Carbine Replacement: Enter the M5

For our weapon, which we’ll call the M5, we’re going to switch to a familiar, but internally different platform: the Heckler and Koch 416 assault rifle. The HK 416 is externally nearly identical to the M4 carbine, aside from some cosmetic changes, but internally uses a gas-piston operating system that requires less frequent cleaning. The similarities between the two platforms will make transitioning to the new carbine for existing troops easier.

The modular system of the M5, which breaks down into upper and lower receivers, will allow the Army to make future changes—such as caliber changes or trigger group improvements–possible without buying a completely new weapon. This will make upgrades easier to implement in the future.

Another key change: switching to a new cartridge. The M5 will be chambered in 6.5-millimeter Grendel. The 6.5 Grendel round offers improved range and lethality over the existing 5.55-millimeter round without utilizing fundamentally different technology (such as cased telescope cartridges, polymer cases, or fully caseless ammunition). The M5 would use the same 14.5-inch long barrel as the existing M4 carbine, ensuring that the weapon is manageable in the enclosed spaces of a truck, Stryker armored vehicle, or M2 Bradley infantry fighting vehicle.

The Army would purchase the M5 stock without optics or aiming devices, an a la carte solution that recognizes a permanently attached optic–such as the optic on the German Army’s Heckler and Koch G36 assault rifle–is unlikely to be ideal under every circumstance. Instead, the M5 would feature M1913 Picatinny rails at the three, six, nine and twelve o’clock positions outside the barrel and along the top of the upper receiver. This would allow the Army to change the optic depending on the circumstance and make future upgrades easier.

One of the best things about our weapon design is that it’s actually the M4 carbine! The U.S. Army could have this new carbine by replacing the upper receiver on existing M4s with a HK 416 upper receiver chambered in 6.5-millimeter Grendel. This recycles at least half of the carbine, saving acquisition costs over buying an entirely new weapon. The M5 externally would look almost identical to the M4.

There are a few downsides to the new weapon. A 6.5-millimeter Grendel weapon would require new ammunition magazines and vast new stocks of 6.5-millimeter ammunition. Furthermore, the magazines would carry 24 rounds of ammunition, six rounds less than a 5.56-millimeter ammunition magazine. That’s an inherent tradeoff of having a slightly larger, more powerful round. The new round would also be a non-standard NATO round and incompatible with rifles and carbines fielded by our allies–most of whom show no intention of migrating away from the 5.56 cartridge.

M4 Carbine Replacement Without Breaking the Bank 

In choosing a new carbine, the U.S. Army must navigate a maze of requirements, prioritizing them and likely leave some of them unfulfilled in order to field a rifle. It must also recognize that against the backdrop of more expensive modernization efforts, such as the effort to produce next generation helicopters and infantry fighting vehicles, procurement budgets in the 2020s and 2030s will be tight and there will be a temptation to “make do” with the M4. The new weapon must be as budget friendly as possible while still offering enough improved performance to justify the expense. The “M5” is such a weapon, offering improved lethality and range without breaking the bank.

Kyle Mizokami is a defense and national security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch.

Written By

Kyle Mizokami is a defense and national-security writer based in San Fransisco. His work has appeared in Popular Mechanics, Esquire, The National Interest, Car and Driver, Men's Health, and many others. He is the founder and editor for the blogs Japan Security Watch, Asia Security Watch and War Is Boring.



  1. Kurt Porter

    October 26, 2021 at 10:33 pm

    I would totally agree the m5 as mentioned would be easy to learn cost affective replacement for the m4 since your just buying the upper in 6.5grendel don’t see any negatives in this proposal wish things were as easy as pie as this would be

  2. IAC.

    October 27, 2021 at 7:47 am

    The Direct Gas system on the AR-style rifle isn’t as jam sensitive to being dirty, as you allude to.
    A daily wipe off of the bolt & chamber, and bore snaking the barrel is not difficult.
    A possible alternative to the 6.5 Grendel, is a 6x45mm round, i.e expanding the current round to 6mm, and loading with a 90 grain bullet.
    Nothing on the current rifle needs changing, except swapping the 5.56 barrel to 6mm, and selling off the 5.56s.

  3. Adam

    October 27, 2021 at 11:58 am

    Bolt life is a weakness of the 6.5 Grendel. It won’t ever be a service round in the AR15 platform.

  4. Kas

    October 27, 2021 at 12:42 pm

    The problem with staying with the m4 platform is that there is an inherent limit to a cartridge’s performance based on case volume, and there is only so much you can increase that performance without increasing the size of the action.

    Without increasing case volume, you generally have tradeoffs such as increasing armor penetration at the cost of lethality or increasing terminal performance at the cost of velocity and trajectory. With improvements in ammunition we’ve hit the upper limit in performance of the 5.56 round and no other round for the AR action, 6.5 grendel included, can offer enough of an improvement without a tradeoff in other areas to be worth the cost of a switch away from the current platform.

    The reason the NGSW program exists is because the m4 and m16 platforms have proven ineffective at penetrating body armor at long range. By 500 yards, 6.5 grendel has lost enough energy that it’s approaching handgun levels of potency, and is no more capable of piercing body armor than 5.56.

    In order to deal with near-peer forces equipped with body armor, as well as insurgent forces with the same, we need a round with a larger case volume, and a correspondingly bigger action. 6.5 Grendel as well as 6.8 SPC were already considered by the military years ago and found to be an insufficient upgrade to be worth it.

  5. William Harkins

    October 28, 2021 at 12:02 am

    IAC. Is 100% correct about fouling and needing to be cleaned more often. All weapons need at least regular field maintenance and for some reason this incorrect rumor about the 16/4 needing cleaning more often then most other weapoms in st completely true. It’s a myth thats been perpetuated since before the M16A1 finally changed to a bolt with chrome lining and bolt carrier with chrome lining. Actually at first the entire BCG was completely chromed and that fixed the fouling and jamming problems.
    Also saying the M855 defeats present body armor because kf the steel rod in also false. Kt will defeat level 3A pistol rated body armotpr but level 3 and 4 will stop the M855. Actually the vietnam are 55gr M193 is the round that will defeat current level 3 ballistic plates.

    Im also confused. About 3 months ago I read a article in Military Times saying they were going with the 6.8 SPC round. Was that incorrect reporting or was there a recent change to the Grendel?

  6. Dick

    October 28, 2021 at 12:53 am

    This is so inaccurate. It’s gonna be chambered in 6.8 replacing the 5.56 round. It will NOT have rails at the 3,9, & 6 o clock positions. Telescoping carting IS a casing it is not caseless. Do more research whole.

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