Time for me to tear down a childhood hero. Time for me to expose that the emperor has no clothes. PDWs suck. They’re pointless. They’re stupid. They don’t serve any good purpose and oughtn’t be procured. Let’s define our terms. Personal defense weapons (PDWs) are generally considered to be a class of submachine guns like the FN P90. They’re about as big as a regular pistol-caliber submachine gun, and fire a round that’s designed to be tiny but still penetrate soft body armor.

The FN P90 was introduced in 1991. It was supposed to be issued to “rear echelon” troops, people like clerks, mortar gunners, and vehicle drivers. People who might get in a firefight, but whose primary duties don’t include being very proficient with an assault rifle. This idea is not without historical basis. The M1 Carbine filled basically the same idea, and was well liked by American troops, since it was smaller, lighter, and just generally handier than an M1 Garand or M1 Thompson that frontline infantry usually carried. The P90 was designed to build on the success of earlier submachine guns like the HK MP5, but it was chambered in a new bullet. Instead of shooting already-standard 9x19mm Parabellum, the P90 fired a new 5.7x27mm round that was designed to defeat the soft body armor that Soviet troops were starting to get issued. There were, however, a few problems. First, in that same year, the Soviet Union collapsed, so all those European governments madly slashed defense spending and proceeded to sing kumbaya in a big circle that would eventually be called the EU. Nobody wanted to buy a new gun that needed a whole new non-standard bullet. And there, FN ran into more problems. First, everyone knows that if you want a round to be a standard NATO thing, you have to get America on board. Both 7.62x51mm and 5.56x45mm were American designed rounds that spread¹ to the rest of NATO. And not only did FN fail to get America on board, but before FN could lock up a bunch of contracts, HK brought out the MP7, with it’s own itty bitty high velocity round, the 4.6x30mm. Now, there was a question of which standard to go for.

Meanwhile, the window on the concept’s usefulness was rapidly closing. Other developments would come in and make the P90 and MP7 obsolescent and pointless. That development was the M4 Carbine. This was introduced in 1994, and led to a whole series of what we might term assault carbines, if we liked to classify things. Once the US Army showed the success of the concept, other manufacturers followed suit and introduced short-barrel versions of their existing assault rifles. To be fair to previous engineers, there were a large number of compact versions of the M16 developed for close quarters battle in Vietnam. However, these weapons weren’t the most reliable, and tended to be special forces only. The M4 was a refined and reliable execution of the concept, and saw widespread issue to regular, second-line troops. Eventually, it actually came to replace the M16 for general issue; soldiers in the US Army are issued M4s almost exclusively. For those of you wondering if we can go shorter, we surely can. There’s the Mk. 18 CQBR with a 10.3 inch barrel (instead of the M4’s 14.5″ barrel). Other weapons like the G36C and the AKS-74U also have very short barrels. And that brings a pretty big set of nails in the coffin of the PDW.

To see why, let’s take a look at the big picture first. The compact carbines like the Mk 18, G36C, etc. all share the vast majority of features with their larger parent designs. That means armorers don’t need training to work on a new design, brand new contracts don’t have to be inked, a whole new set of spares doesn’t have to be stocked, and the army doesn’t have to have yet another caliber. All these things are positives that directly affect the bottom line. They also improve combat effectiveness; everybody can use the same magazines firing the same ammunition. Plus, even out of a short barrel, assault rifle rounds like the 5.56 or 5.45 are a lot more effective than 5.7mm. The 5.7mm does penetrate soft armor, but most armies are issuing hard plates now, so that’s of limited value. And what good is penetrating the armor if the terminal effects suck? And 5.7 has atrocious terminal effects. It does a very poor job of getting bad guys to stop what they’re doing and die. Against unarmored threats, conventional pistol rounds like 9mm perform far better. Put another way, 5.7 fires a bullet about two-thirds the weight of a 5.56 round at half the muzzle velocity. And we’re having arguments about whether or not 5.56 is effective enough! So, why would anyone use a much lamer version of it?

The proponents of the PDW will argue that I’m missing the point. Since PDWs are designed for “Second Line” personnel, we can assume that these personnel will not be very good at shooting, and will miss often, so we should give them a weapon that is light and has lots of bullets. The loaded P90 doesn’t give up much weight or bulk on a compact carbine, or even an M4. And spraying wild panic fire is a sign of poor training. But even if we were doing that, assault rifles were designed to perform the same function: to provide conscripts with effective automatic firepower that could also reach out to the longest normal infantry combat ranges (3-400m). Conscripts aren’t very good at shooting, but we gave them effective rounds anyway. Bullets should do their jobs if we (or luck) do ours. We got rid of submachine guns for general issue a while ago because they don’t do anything that a carbine can’t, and submachine guns at least share rounds with pistols. An M4 or Mk. 18 can spray and pray with the best of them, and it’s round is significantly more effective than either 9mm or 5.7mm. And while the 30 round standard assault rifle magazine is smaller than the 50 rounds in the P90, “second line” personnel can share 5.56 with the “front line” personnel. They can’t share 5.7. The logistics gains plus combat efficacy if those rounds hit mean that the smart buy is the carbine.

Sorry, Stargate fans.

1.) Some might say were forced, but not I.