Really Not an M16 at All: Colt’s M231 Port Firing Weapon

The M231 Port Firing Weapon was developed in the 1970s as a part of the M2 Bradley Fighting Vehicle Project. A modern relative of the WW2 Krummlauf, the weapon was intended to provide close-in firepower against infantry that might attempt to overrun the M2. It has no sights or buttstock, and fires from an open bolt only as 1100-1200 rounds/minute. It is intended to be used with M196 tracer ammunition to aim. Early versions were issued with rudimentary sights and a wire collapsing stock (akin to that of an M3A1 Grease Gun), but the weapon proved so difficult to control from the shoulder that the stocks were discarded and policy changed to dictate that the guns never be removed form the Bradleys.

The unique fitting on the front of the hand guard is a coarse thread to screw the gun into the Bradley’s firing port sockets. The fire control system is entirely different form a standard M16, with the hammer being removed entirely in favor of a submachine gun like dropping sear. The recoil system was also completely changed, and in the M231 consists of simply three separate recoil springs nested inside one another.



  1. Medal of Honor Awardee PFC Ross McGinnis C-1/26 INF 1ID used one in Iraq where some units which still had them issued them as HMMWV turret gunner close in weapons.

  2. I remember seeing these on the cover of various RPG books in the late 80s. The sort of post apocalyptic games where people had to scavenge firearms, although it wasn’t explained how someone chose one of these versus a standard M16, given the option.

    The term bullet hose springs to mind, and I agree, it’s a fascinating part of AR history. Didn’t know it was open bolt until now, though.

    • The idea was that these were taken, or to say scavenged, out of broken down apcs, that in apocalyptic times lacked maintenance to get them running. And I suppose that were the only available guns given the times, so there was not an option between it and M16.
      Staying in fantasy mentality, these high rof, no sights full auto only guns would be highly undesirable for any survivor (scarcity of ammo, lack of long range precision), and could serve only for desperate close encounters.

  3. This was a bad idea in hindsight. Since when has any armored vehicle been swarmed by enemy soldiers after Vietnam? The idea of yanking hatches open on modern infantry fighting vehicles in order to insert explosives ended when vehicles operated in groups supported by infantry. Sure, a tank has blind spots but I don’t think anyone would be stupid enough to sit perfectly still for destruction by tank hunter infantry. Swarming the vehicle has become a suicidal task for idiots. I read a fantasy version of such an encounter pitting orcs and goblins against a company of Japanese Type 74 tanks which had been upgraded with urban warfare kits. The fantasy monsters were slaughtered by canister rounds and the coaxial machine guns. Did I mess up?

    • Whats with the “it would never happen”? It most certainly could happen. Not that these 231s would help much, or that a team couldn’t attach a shaped charge to the outside, or an antitank mine underneath and se it off, but whats with the “not possible?

      • Well, has anyone been dumb enough to send tanks and vehicles alone into a ruined city without air support or infantry support apart from Yugoslavia? And swarming only works on a single vehicle stricken by power failure or bad visibility in a very tank unfriendly environment like urban ruins! It can happen but usually only due to stupidity and bad planning. Does every infantry platoon have a convenient giant stash of mines? I think not. The whole point is not to play reactionary but to account for the possibility before it happens. So don’t send your armored car into the deep dark woods alone. You may never see it again.

      • After the initial slaughtering the monsters were too scared to try swarming the tanks again let alone clog the tracks. And the tanks were no longer in danger by that point, having been reinforced by a larger group of soldiers and vehicles. Let that sink in.

    • “tank has blind spots”
      Possibly designers of M2 Medium Tank
      feared that and thus this tank gained several machine guns, even for inter-war period standards (4 x sponsons-mounted, 2 x glacis-mounted, 1 x co-axial with 37 mm gun, optionally: 2 x pintles-mounted for AA defense, together up to 9 machine guns)

      • Just pick one or perhaps both already. The whole point is that swarming a tank with infantry is generally a BAD idea should the tank not be one or more of the following:

        1. Devoid of sufficient crew and/or support from its allies
        2. immobilized/engine-killed
        3. unable to operate internal armaments
        4. ON FIRE

    • I was under the impression back in 1982 that the FPW was for suppressing fires as the Brad came onto the objective BEFORE the squad dismounted. At least that’s what we were told in 2nd Armored division

      • The M231 was more a “Keeping up with the Jones’s” sort of thing than it was a well-reasoned response to the issue of mechanized warfare. The BMP, you see, had firing ports and the ability for the AKMS to be mounted in them. Never mind that the whole bloody concept of an Infantry Fighting Vehicle makes no damn sense whatsoever, the Russkies had the things, so we must, as well.

        Part of the reason they went away was the ODS version of the Bradley, which did away with the idea of outward-facing seats, going to the inwards-facing bench seats. Doing that made debarking the troops easier, loading quicker, and made a lot more sense. Because few people can look backwards through the periscopes and fire, the M231 and the whole “firing port defense” made zero sense after they rationalized the troop-carrying compartment.

        To this day, I remain highly dubious of the entire IFV concept. Putting the troops into the same vehicle with the support weapons makes little to no sense, because in my observation, the places from whence you can best employ those weapons are rarely where you need to drop the troops off for their role in the battle. Likewise, why the hell give the commanders the temptation to go haring off and play “Joe Cool, Tank Ace” with the vehicle that’s also carrying around the dismounts? Those dismounts are entirely extraneous to the armored vehicle battle, and just out there hanging to run up the casualty count when you discover, much to your chagrin, that a thin-skin Bradley ain’t the vehicle to take up against a platoon of T-72 tanks in a close-quarters tank-on-IFV fight.

        I think the BMP is a stupid concept, which was proven to be right on multiple occasions since it was first issued, and that we were equally stupid to copy it. Troops do not belong in vehicles intended to participate in vehicle-on-vehicle combat; they add nothing to that fight, do not belong there, and should be aboard dedicated transport vehicles that do nothing but carry them to the point where they are effective and useful. The whole idea behind the BMP was based on a flawed and fantastical concept of what the modern battlefield would look like, chemical- and nuclear-contaminated. Had that ever come to fruition, I suspect that the BMP would have been most useful as a container for collecting up the contaminated remains of the assaulting Soviet forces, and then serving as a handy thing to inter them in. Having had the dubious pleasure of riding around in a BMP for short stretches, I suspect that most of the poor bastards having to try to fight from those things on contaminated battlefields would have wound up killing themselves to end the pain and suffering caused by just riding around in those things while in a chemical suit…

        • So much groupthink and bureaucratic inertia in the concept of these things in the first place, as well as their execution —

          So if the weapon is supposed to be used chiefly for suppressive fire, and is going to be difficult to aim in any event, we want an extra high rate of fire. Okay, but why not then also supply extended or drum magazines so that the shooter has more than 1½ seconds of fire? That’s barely enough time to adjust your tracer stream. I’d imagine they were concerned the troops would make off with them for their own weapons.

          Why the elaborate multi-spring recoil-buffering system for a weapon that’s mounted in a fairly rigid ball mount? You’d think that’d be about the most stable firing platform there is.

          • The buffer is how it is to make it more maneuverable in the enclosed fighting compartment so you can swing that little bastard around to the limits of the firing port interface! Also the carry handle isn’t even machined to accept the installation of a rear sight or a carry handle scope mount for some really odd reason.

            It also uses a very nonstandard bolt carrier group which includes a cylindrical striker. Additionally the recoil spring array is very neat all on it’s own with 3 nested springs (make sure each spring coils in the opposite direction than the one next to it or bad things happen!) And the combination of a polyurethane bumper and guide rod in the very beefy threaded end cap of it’s buffer tube.

            And last but not least, the m231 is only for use with the old M193 matched tracer ammunition not the tracer ammo matched to m855!

    • Not to be too cynical about it but the main motivation for Bradley Firing Port Weapons (and firing ports on Bradleys) was the firing ports on the Red Army’s BMP-1 IFV.

      Interesting that the Soviets used the Soldier’s standard AKM with a couple accessories rather than an additional new weapon

      I have a July 1978 technical report titled, TECHNICAL RISK ASSESSMENT OF EXTENDED CONFIGURATIONS OF THE M113AIE1 that was a series of reasons why an extended version of the M113 wouldn’t substitute for what became the Bradley; the report says that the firing port thing was “mission essential” so a modified M113 APC was a non-starter.

    • It was due to “BMP envy”. The US Army became aware of the Soviet BMP (now known as BMP-1), with its firing ports for the Soviet motorized riflemen.

      There was no good way to make a firing port for use with M16A1 rifles, due to length and space constraints, so they came up with the idea of cut down “carbine” sized “Firing Port Weapons” that stayed in place.

      Unfortunately, they realized there was no good way to aim them, due to space constraints, so they decided on the “full auto only, with straight tracer” concept, which drove open bolt operation and a high ROF.

      Yeah, it was a stupid idea, but it’s easy to see how they arrived at it, given hat they knew, what they feared, and early 1970s thinking.

    • The Bradley was designed as our “Oh Shit!” response to the BMP-1, and the duictate was to duplicate or exceed *every* feature on the BMP that differenciated it from earlier APCs like the M113.

      Since the Russians could fire their AKs from teh firing ports, we “had” to have that capability… only there isn’t room to bolt an M16A1 into any feasible firing port mechanism that could be incorporated, due to the geometry of the rifle. Aks have shorter barrels, coult latch in fatrher up the barrel group (the bracket grabs it at the gas tube and barrel, behind the gas block), and the BMP design team assumed motorized riflemen wiuld have the folding stock version.

    • It was a reaction to the fact that Soviet soldiers could use their folding stock AKs with firing ports in the BMP, while the nature (length, basically) of the M16 precluded that capability from the Bradley.

      Pretty much, the Bradley IFV and CFV were designed to meet or exceed every real *or imagined* feature of the BMP.

  4. The major issue with the M231 was mission creep. Qualification tables went to 300 meters–after all, it was a 5.56mm! In reality, anything more than 15 meters was pushing things, though a skilled gunner with quick trigger finger could walk rounds on target. At 1200 rpm cyclic rate, the 30 round magazine would last 1.5 seconds–not a lot of reaction time.

    I was “armorer trained” on this weapon at Fort Riley, Kansas–but only by reading the manuals. None were available for the class even though Fort Riley was supposed to have them on hand for the new Bradley. No Bradleys, no M231. As for firing them from a bouncing Bradley, there was precedent–sponson guns on the old WWII M-3 tank and some early Soviet T-54 tanks had a gun fixed to fire forward only.

    • “old WWII M-3 tank”
      Vague statement; please chose exactly one option:
      – M3 Medium Tank
      – M3 Light Tank
      Machine gun with fixed rotation axises in relation to vehicle has Russian name: курсовой пулемёт; in Russia such style of machine gun fell out of favor in tank usage, but can be still found on some APC namely BMD-1 and BTR-D (two forward firing PKT) and also BMD-2 (single forward firing PKT).
      U.S. World War II-era M6 Heavy Tank was has single GUN, machine, cal. 30, M1919A4, in bow of hull, there is not way to traverse it other than rotating whole vehicle, but it can be elevated (depressed), see 13th page in this pdf: for more data

      • M3 medium tank. It had TWO fixed aimed forward for the driver to fire. Mostly they were deleted and the space used for storage.

        • HOLD IT. The M3 Light Tank (Stuart) was the one with two fixed machine guns for the driver, NOT the medium tank (Lee). TRUST ME, I’m looking at the specs RIGHT NOW.

  5. I think it was less a fear of being swarmed than having the ability for fighting while inside the vehicle,Probably because BMPs had firing ports.

    • Pree-cisely…

      Keeping up with the Jones’s, mostly. Never mind that it just doesn’t work in the real world. There’s nothing quite as much fun as sneaking up on a Bradley platoon, buttoned-up and over-confident on an objective, and hanging satchel charges off the damn things while they uselessly fire away at you. Although, the fact that we could do that might also have been due to the piss-poor MILES gear they had hanging off them…

  6. The book “The Bradley and how it got that way” by Harworth ( is an excellent, if depressing, tale of how the M2 and M3 Bradly fighting vehicles came to have features like port firing weapons. A special weapon, the M231, was adopted over the M16A1 because the screw in collar had better potential to prevent NBC contamination than a clip on arrangement like the Soviets used in the BMP-1. The idea as used by the BMP, and copied by the Bradley, was that the infantry could fight from the vehicle and not have to dismount in a contaminated battlefield. In the 1970s it was assumed the U.S. Army would be fighting in a NBC environment in Europe against the USSR. Hunnicutt’s “Bradley” covers the development of the concept from the U.S. perspective in great detail (

    Interesting video, thanks!

  7. The first tanks in the First World War had pistol ports. When Germany re-armed, their early tanks also had pistol ports (does the German MK I and MK II rate a ‘forgotten weapons’ entry?) and the MP-38 was designed with a barrel rest and stud to allow firing from the German armored half track in relative safety–the stud helped keep the gun from falling inside while shooting and wiping out passengers and crew. The Soviet personnel carriers post-war had both firing ports and their hatches were designed to allow personnel to fire over the sides with some degree of protection from small arms and artillery splinters. Germany’s Marder infantry fighting vehicle was provided with improved firing ports and modified UZI submachine guns–the UZI would screw into the firing port and permit mounted infantry to fire on the move from under armor and protected from radiation and poison gases and from biological weaponry.

    There are pros and cons to these solutions. A submachine gun fires pistol bullets and won’t punch through light cover, sometimes even failing to penetrate standard infantry combat equipment. These firing ports were designed for close quarter use, the equivalent of fixed bayonets for tankers–and low-penetration bullets did permit “delousing” by neighboring armor crews shooting the swarming infantry off the tank.

    When infantry anti-tank weapons evolved from clumsy cannon and anti-tank rifles and demolition charges and field expedient flame weapons (aka Molotov Cocktail) to the bazooka, PIAT, Panzerfaust and anti-tank rifle grenade, infantry usually didn’t have to employ the desperate tactic of swarming on enemy armor to place a bomb inside or shoot through vents and vision slits and open hatches. Countering the anti-tank guided missile (the dreaded SAGGER of the Yom Kippur War) demanded guns that could reach out several kilometers instead of perhaps 100 meters–and another Bradley weapon system was used for the long range SAGGER crew kills–the Bushmaster 25mm automatic cannon. Yes, there was that M240 co-ax machine gun, the most-used gun on the Bradley, and Bradleys carry a pair of TOW missile launchers on the turret and a total of six TOW missiles (if memory is correct)–but the 25mm gun has more HE than AP, there’s a fire selector to pick between the small magazine of AP and larger magazine of HE, and the intended target is the guided missiles aimed at the Abrams tank.

    While it’s cool to look at the mechanical aspects of the gun, the Big Picture includes how the gun fits into the tactical scheme and the logistics involved (which includes operator training, gun maintenance, and the qualification standards). The US Army didn’t intend for their Bradleys to wander down narrow streets in towns and cities on solo operations–where improvised weapons could immobilize and destroy the Bradley and crew. The M231’s fields of fire to the flanks were most useful for delousing other Bradleys and for protecting Abrams tanks in open country–but lacked enough depression or elevation for vehicle self-protection. The rear door guns were a little better but still limited in elevation, depression and transverse–they had a very limited field of fire. When immobilized, the “dead space” were Bradley weapons fire didn’t reach were how enemy infantry could get close enough to harm the passengers and crew using field expedient weapons.

    If you get the opportunity, tour the Maginot Line and look at the firing ports in that fort–note how there were still dead spots. Tanks had the ability to move. Forts didn’t. Of course, the Maginot line had grenade chutes built in to take care of that eventuality.

    • “German MK I and MK II”
      What is that? U.S. codename for Sd.Kfz. 101 and Sd.Kfz. 121 xor something different?

      • Panzer 1 and Panzer 2, both are light tanks intended to support infantry. Panzer 1 has 2 machine guns in turret. Panzer 2 has one automatic cannon and the coaxial machine gun. The turret armaments were not capable of dealing with infantry mob ambushes let alone angry Polish horsemen who got close enough to attack with grenades or anti tank rifles…

  8. The raison d’être of this gun was to provide adequate sealing for firing ports. In the eighties a NBC battlefield was a probable scenario and tight adjustment was required for firing port weapons.

  9. I’ve seen photos of the initial version with the wire stock and flip up front sight. It might be fun to retcon a modern AR to (mostly) look like one.

  10. The two major problems with the M231 were first of all, the six or seven FPWs stowed internally added to the clutter in what was already a fairly tightly-packed vehicle. (The available “crew space” of a Brad actually isn’t noticeably more than that of an M113.)

    Second, by the time the Bradley actually entered combat, the firing ports were mostly gone, having been covered by first augmented spaced armor, and then by reactive armor stacked on top of that. RPGs fired from 100 meters in ambush were determined to be a much more serious threat than a Soviet-style infantry “swarm attack”. (Generally, jihadis like to do things the easy way, and RPGs are a dime a dozen in the sandbox.)

    Incidentally, the main reason M231s basically disappeared from the inventory is that most of them got sent back to Colt to be the basis of the first run of M4 carbines, with a new firing assembly (standard hammer type, selective fire), an actual buttstock (retractable type,) and a completely new “flattop” upper receiver with a (slightly) longer barrel.

    Yes, this is why those early M4s have much lower S/Ns than the second batch and later. They were built on receivers that first came out of Hartford in the late 1970s and early 1980s.



    • Converting the M231 port firing weapon into the M4 carbine would have been cheaper and better if they simply scrapped the receivers and stamped the old serial number on the new rifle. I would be inclined to doubt the story if not for two rifle projects: the 1870/1873 “Trapdoor Springfield” and the M14. The Springfield in 50/70 (1870) and 45/70 (1873 and subsequent marks) was supposed to re-use the hundreds of thousands of Civil War Springfield muzzle loading rifles with a “simple conversion.” A selling point of the M14 over the FAL (T-48) ( ) was that the M14 could use Garand machinery. The Italians did this with their Beretta BM59 series ( )
      but the M14 had minimal parts exchange–didn’t even simply adopt the M4 bayonet of the M1/M2 carbine but a new M6 bayonet of similar design that wouldn’t interchange! Okay, could use the Garand’s sling and a few other parts. Minor parts.

      Perhaps it was a funding game with Congress. There was the B-50 bomber (originally the B-29D) and the THREE models of the F-84 fighter (the swept-wing fighter was supposed to have a different model number, and there was an unsuccessful turboprop edition).

      I conclude that weapon selection is 90% politics, 9% logistics and 1% weapon system performance. While the US Army struggled with the Trapdoor Springfield system, the Navy and Marines had the superior Remington 1868 Rolling Block Rifle.

    • Brads have *less* “crew space” than “Mike-One-Thirteens”. You cannot stick an entire nine man dismount squad into a Bradley, yet that was *standard* for the M113. (Standard squad when the M113 was new was eleven men. Nine dismounting, with two guys left behind in the carrier to drive and shoot the .50 in overwatch.)

      Plus, the M113 had room to carry a couple of “overflow” bodies.

  11. The Port Weapons were used in ’91 by 48th Brigade dismounts at Fort Irwin. They were used to experiment with various assault strategies against mock ups of Iraqi trench systems. The final conclusion was to avoid trench fighting altogether and use tanks with mine plows to collapse the defenses. Still the M231 was used in a live fire submachine gun role for those practice assaults.

  12. I guess I’m one of the few lunatics to have ever carried an M231 as their personal sidearm in combat. During Desert Storm, I served with 1/34 Armor, 1st Bde, 1st ID. I was supposed to be the Battalion Commander’s Staff Humvee driver, but during the ground war while the CO was on his tank we were used as scouts, dispatch riders, and rolling mine detectors. We knew ahead of time that one of our jobs would be to search and clear enemy bunkers since we had no assigned grunts and only a few dismounts on our six scout Bradleys. My boss went to his neighbor, the Infantry commander and asked if they had anything that would help with the down and dirty work. He loaned me one of their M231s and a crate of pure tracer rounds to feed it. It is one of the most un-ergonomic and beastly weapons I have ever handled, but when fired in a confined space it is truly terrifying. I only had to pull the trigger twice in anger, but both times it ended the argument before the last shell casing finished ricocheting around. This is actually a blessing because changing magazines is about as awkward as nearly every other aspect of this gun. Despite its many flaws it remains one of my favorite weapons.

    • I recall the FPW wasn’t rifled in the normal way, and didn’t properly stabilize the 5.56mm bullets by design, did you notice this when using it (odd groupings, keyholing), or did you shoot it at so close a distance it wasn’t noticeable?

      • “rifled in the normal way?”

        If I remember correctly, the M231 had the same 1-in-12″ twist as the then-standard M16A1 rifle for the M196 tracer round (this was before the M16A2 and its 1-in-7″ twist designed to stabilize the long M856 tracer round was adopted). Yes, TRACER–Arctic testing demonstrated that the M196 tracer wasn’t stable (had excessive yaw) through the original 1-in-14″ twist of the early AR-15 select-fire assault rifles when tested at low temperatures as noted by the wide projectile groups at distance. M193 projectiles (5.56mm and 55 grain boat tail full metal jacket) had adequate stability and accuracy, but the longer M196 tracer didn’t.

        My battalion switched to the M16A2 in August 1987 and one issue was that the M193 and M855 ball rounds were not fully interchangeable. The M16A2 could fire either the old ball or new ball and exhibit only a point-of-impact shift (there wasn’t enough information to predict this shift and the procedure was to zero with either 55 grain or 62 grain ammunition and then qualify with the same bullet weight out to 300 meters) but the M16A1 had two issues: the POI shift, and then the group size with the heavier M855 projectile would become excessive beyond 100 meters. The bullet did NOT tumble through the air–more like wobble through the air. At very short distances, probably because conventional rifling vents propellant gases ahead of the bullet (a little), or perhaps because the bullet isn’t gyroscopically stable yet, hits in ballistic gelatin exhibit the bullet destabilizing and then tumbling through the gelatin block–at longer distances such as 200 meters plus the bullet drills through with minimum yaw and maximum penetration. Field Manual 90-10-1 (urban warfare) has a maximum penetration table for small arms and both 7.62mm and 5.56mm NATO exhibit maximum penetration around 200 meters–not 20 meters. The keyholing effect noted on paper targets is because the bullet strikes target paper at a shallow angle and then actually tumbles–even the mighty 7.62x51mm NATO M80 ball will tumble when it hits flesh despite being fully stabilized through the 1-in-10″ rifling twist and having a bullet that is almost three times the mass of the 5.56mm. The yaw is more of a wobble than a tumble or the range of the M16 would have been less than the revolutionary war musket unless spherical projectiles (the REAL bullet) were used. Just try to get high speed movie footage of these bullets in flight at several places along their trajectory! The keyholing “tumble” takes place at the target and beyond, not before–else the projectile would run out of steam from flying broadside and wouldn’t be “broad side of a barn” accurate beyond BB gun distance.

        Sport shooters like the 1-in-9′ twist for 5.56mm match rifles because it optimally stabilizes the M855 projectile–the 1-in-7″ twist is for putting the M856 closer to the same POI as the M855 and under severe cold conditions (something below zero, I’ve forgotten the data). As sport shooters don’t do tracers, they wring maximum performance out of their platforms.

        Active duty Soldiers and Marines have limited ability to conduct testing of their basic tools (the M16A2 rifle and M4 carbine) and mostly have to take their weapon capabilities and limitations as a matter of faith. When many units only use the 1000-inch Alt C qualification course to save training time and ammunition (and money) instead of the 50 to 300 meter standard Army TRAINFIRE course or the older Marine Corps KD course (at 200 meters, 300 meters and 500 meters) and live fire under simulated combat conditions is rare, rumors replace the distrusted “facts from the brass.” Getting the facts is difficult because it’s not really important that cannon fodder be educated–even today.

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