Craig has done a fine job of showing us some of the early attempts to replace the M113 that eventually led to the M2/M3 Bradley family of vehicles. Concurrent with that development was a changing doctrine about how mechanized infantry should fight.
When the Soviet BMP-1 infantry fighting vehicle first appeared, US observers were stunned. The Soviets had taken a vastly different approach to mounted combat than the US, and it was a better one. For years before the appearance of the BMP in 1967, the Red Army had concentrated on developing its BTR series of wheeled armored personnel carriers. While the choice of wheels over tracks could be debated, the actual employment of infantry from them was very analogous to the way the US Army intended to employ its M113 mounted infantry. That is, the armored personnel carrier was primarily there to transport an infantry squad to the battlefield, and protect if from artillery fire en route.
The BMP, however, was designed to accompany tanks in the actual assault on the unit’s objective, and to actually allow its squad to remain on board the vehicle and fight from within the vehicle. In addition to the BMP’s 73mm gun and 7.62mm coax machine gun (and it’s AT-3 Sagger anti-tank missile) the BMP had a series of ports in its sides that allowed the infantry squad on board to use their weapons to add suppressive fires. This was pretty much a “spray and pray” approach. Lots of lead flying around was designed to keep our infantry down in its foxholes and prevent us from taking shots at BMPs with light anti-tank weapons. After the tanks and BMPs had crossed the objective, the Soviet infantry could dismount and mop up our positions by attacking from the rear. Importantly, the Soviet riflemen would be protected from artillery and small arms fire for almost the whole attack.
The US Army decided that was a pretty good approach to mounted combat. So one of the important properties of the successor to the M113 was that it too needed to have firing ports. One small problem though. Unlike the Soviets, who used a folding stock version of the AKM rifle, the US had just adopted the M16 series rifle, and there was just no practical way of handling such a long rifle through a firing port in the cramped interior of an armored vehicle.
Well, the Army wasn’t going to allow something minor like that stand in the way of a doctrinal decision that had already been made. Mounted assaults were the way forward, and the Army would be able to fire from firing ports. So the call went out to actually develop a weapon that would be able to fit in a port and small enough to be handled inside an armored vehicle. So in addition to trying to develop a new armored vehicle, the army had to look to industry to develop a new weapon to go with it. There were a couple different candidates, including a modification of the old M3 “Grease Gun” .45cal submachine gun. In the end, the Army adopted an extensively redesigned M16 action called the M231 Firing Port Weapon (FPW).
The M231 has a much shorter barrel than the M16, and also doesn’t have a front sight. You’ll notice at the front, it is designed to screw into the ball joint of a firing port. In addition, because it is intended to be fired form the port, it has no buttstock, just the buffer tube to hold the action spring. But the most significant changes to the weapon are internal.
Whereas the M16 series rifles fire from a closed bolt, and are selective fire, the M231 fires from an open bolt, and is only capable of fully automatic fire. It also has a much, much higher cyclic rate of fire than the M16 series. Instead of firing at about 600 rounds per minute, the M231 fires at about 1,100 rounds per minute. That little 30 round magazine doesn’t last long.
Originally, there were to be six firing ports on the infantry variant of the Bradley, the M2. There were two ports on each side, and two ports in the rear ramp of the vehicle. Combined with the turret weapons, this would notionally give the Bradley all around suppressive fire. The need for the squad to be facing outboard while using the M231 dictated the very awkward internal seating arrangement of the Bradley. Each firing port had a periscope through which the soldier was expected to point his FPW and then walk the rounds onto the target. To aid this, the M231 was to be loaded exclusively with the M196 tracer round. And because the M231 fired from an open bolt, a lot of the gun gases would be released inside the vehicle. To counter this, each position had a flexible exhaust hose attached to a fan and a brass catcher bag (hot brass bouncing all over the place inside a vehicle would be annoying at the very least, and potentially very dangerous). Because the FPW uses an all-tracer ammunition load, that meant that troops had to store their regular load of ammunition, and have a separate supply of ammo just for the FPWs. If you look under the seats of early production Bradleys, there is an ammo pouch under each seat to hold magazines for the FPW.
In the end, very few units ever actually used the FPW beyond mandatory training for it. One reason is that the vulnerability of light armor to anti-tank weapons meant that it was almost always safer to dismount the infantry just short of the objective, and support them with fires from the turret using the 25mm gun and the 7.62mm coax. The other big reason was, you couldn’t hit a damn thing with the FPWs. Trying to walk in rounds while looking through a dirty, dust periscope from the back of a bouncing, swaying vehicle just wasn’t very practical. The one time I actually fired one from the firing port, I just waited for the Bradley Commander to yell “fire!” and held the trigger down. I have no earthly idea where the rounds actually went. We had targets staked along the side of a road, but after something like a dozen Brads had made firing passes (at a range of about 10 meters) there wasn’t a single hole in any of the half dozen targets.
As later model Bradleys had to have heavier side armor to counter weapons like the BMP-2’s 30mm cannon, and other anti-armor weapons, the firing ports on the side were blanked out by slabs of add-on armor. The firing ports on the ramp of the vehicle remained in place. But given that they have to be removed before the ramp can be lowered, they were almost never used.
Today, M231 FPWs are still in unit armories, but mostly serve one of two purposes. First, they either gather dust and generate paperwork regarding maintenance and security, or they are used as a last ditch, close in weapon by dismounted soldiers, such as those manning the cupolas on up-armored Humvees. They aren’t very effective in that role, but they sure to shoot fast.