PDWs: More Than Just "Modern Submachine Guns" – What Sets Them Apart?

PDWs and submachine guns may look similar at first – compact size, high magazine capacity, and quick handling. But there’s a critical difference. PDWs weren’t just designed as “modern submachine guns”; they were developed to meet specific military and law enforcement needs. In this article, we’ll break down what truly sets these categories apart, why caliber alone isn’t enough, and how they perform in real-world use.

Is a PDW the Same as a Submachine Gun? A Clear Comparison

The simple answer is no. A submachine gun is a compact automatic weapon chambered in pistol calibers. A PDW is a very similar weapon, but with one key difference: it uses a specific caliber that sets it apart from traditional submachine guns.

However, there’s one important caveat. This distinction has only been clear since the early 1990s. The turning point came with the release of the first truly successful PDW – the FN P90. While the history leading up to this development is fascinating, we’ll focus here on the PDW as we know it today.

H&K MP7A2 equipped with an Aimpoint Micro T-2 red dot sight, tactical light, and forward grip. Photo: SR Tactical / Wikimedia Commons (CC BY-SA 4.0)

What is a PDW and How Does It Differ from a Submachine Gun?

As mentioned, the primary difference between a submachine gun and a PDW lies in the caliber. Submachine guns today predominantly use 9mm Luger ammunition. This caliber is known for its solid performance in a relatively compact package.

However, in the past, 9mm Luger was just one of many options. Submachine guns were manufactured in a wide range of calibers. These included small calibers like .22 LR (American 180), 7.65mm Browning (Sa vz. 61), and various country-specific calibers, such as 9×23mm Largo, 7.65mm French, and 7.62mm Tokarev. The classic .45 ACP, used in models like the Thompson and M3 Grease Gun, was also common.

When we talk about “true” PDW calibers, there are essentially two widely used today. The 5.7×28mm round, developed by FN Herstal, is used in the FN P90 and the FN Five-seveN pistol. The latter is well-known to fans of games like Counter-Strike.

In recent years, this caliber has seen a sharp rise in popularity in the U.S. It has led to the release of additional pistols (such as the Ruger 57 and S&W M&P 57) and carbines chambered in 5.7×28mm. It is even possible to convert an AR-15 to fire this PDW round.

The second alternative, introduced at the turn of the millennium, is the 4.6×30mm caliber. Developed by Heckler & Koch (H&K), it was designed for their compact automatic weapon, the H&K MP7. This caliber is virtually reserved for military and law enforcement use.

PDW Caliber vs. 9mm Luger: What’s the Real Difference?

You might be wondering why anyone bothered to develop a new caliber when there were already proven submachine guns and assault rifles available. The problem was in the balance between performance and size. While submachine guns are typically limited by the performance of their ammunition, compact assault rifles face limitations in terms of size and weight.

The rising threats of global terrorism, organized crime, and the increasing availability of soft ballistic protection called for a solution. This led to the NATO program outlined in document D/296, aiming to develop a personal defense weapon (PDW) ready for adoption by the year 2000.

Special forces, military units, vehicle crews, pilots, and officers all needed a weapon that was extremely compact, lightweight, with sufficient magazine capacity, and chambered for a round capable of defeating soft body armor.

The solution came in the form of a small, lightweight projectile with high muzzle velocity. The round, with a bottlenecked case and a small, light bullet, bears a striking resemblance to a rifle cartridge, such as the .223 Remington, but is significantly scaled down.

But the benefits of PDW ammunition don’t end with its ability to defeat ballistic protection. Not by a long shot. Another advantage is its flat trajectory. In other words, the bullet doesn’t drop like a 9mm Luger does over distances of 50 to 100 meters. In fact, both of the aforementioned PDW calibers still maintain a higher velocity (around 500–600 m/s) at 100 meters than the 9mm Luger fired from a submachine gun like the H&K MP5 (around 400 m/s).

True, the PDW round lacks significant stopping power due to its lighter bullet weight. However, it compensates for this with extremely low recoil, greater range, and its ability to penetrate soft body armor (up to Level IIIA).

FN P90 in use by tactical divers, firing from the prone position. The bullpup design allows for a compact size while maintaining barrel length. Photo: Ayuwoki2019 / Wikimedia Commons (CC BY-SA 3.0)

FN P90 and H&K MP7: Two Different Paths to PDW

We’ve already mentioned two of the most important representatives of the PDW category: the FN P90 and H&K MP7. Despite both being PDWs, these weapons are visually very different, and they also differ significantly in other aspects.

The FN P90 was introduced in the early 1990s and began to be actively deployed around 1994. Although it may not be immediately obvious, the P90 is a bullpup design. This means the action, in this case, a dynamic bolt, and the barrel chamber are shifted closer to the shooter’s shoulder, with the trigger and grip located in front of the shooter towards the muzzle. This layout allows the weapon to have a shorter overall length while maintaining a longer barrel. Spent casings are ejected through a chute between the stock and the grip. This feature also makes it comfortable for both left- and right-handed shooters.

A distinguishing feature of the P90 is its magazine, which is positioned on top of the receiver. The 50-round capacity is achieved by placing the rounds perpendicular to the barrel axis. The mouth of the magazine is equipped with a sort of "spinner" that rotates the round 90° before feeding it into the chamber. This concept was first used by Czech designer Václav Holek in the ZB 47 submachine gun, developed four decades earlier.

Both the receiver and parts of the internal mechanism are made of durable polymer, which contributes to the weapon’s low weight, a key factor for PDWs. The P90’s futuristic design has also made it a fixture in video games, films, and television series.

The H&K MP7, at first glance, appears more conventional, and its magazine is located in the grip, similar to classic submachine guns like the UZI or Sa 26. But that’s where the similarities end.

While we’ve already discussed the caliber, the MP7 offers more than just that. Unlike submachine guns and its main competitor, the FN P90, the H&K MP7 operates with a locked bolt mechanism rather than the typical unlocked dynamic bolt. In terms of design, the MP7 can be seen as a more compact version of the G36 rifle from the same manufacturer.

Like the G36, and in fact, like the P90, the MP7 makes extensive use of polymers in its construction. The magazines, with capacities of 20 and 40 rounds, are more conventional compared to the P90, but the weapon itself is considerably shorter, thanks to a retractable shoulder stock.

A key point for us is that the PDW H&K MP7 is used by the Czech Republic’s police force in its A1 version. It serves alongside submachine guns like the H&K MP5 and the CZ Scorpion EVO 3 A1.

H&K MP7 held in hand – compact dimensions are one of the key features of the PDW category. Photo: Pistoufinaire / Wikimedia Commons (CC BY-SA 3.0)

PDWs and Submachine Guns in the Civilian Market: Availability, Options, and Limitations

When it comes to civilian versions of PDWs and submachine guns, the market for submachine guns is flooded with what are known as PCCs – Pistol Caliber Carbines. These are essentially submachine guns that lack full-auto fire capability. The options are vast, and the selection is broad.

The situation with PDWs is quite different. Finding a semi-auto FN P90 is possible, but don’t expect it to be cheap. As for the H&K MP7, it’s even more limited – it simply doesn’t exist in the civilian market. The last option is the conversion of AR-15 rifles or various U.S.-made PCCs chambered in the 5.7×28mm round.

Before acquiring such a weapon, though, it’s important to consider the cost of ammunition. There’s a huge difference between paying $5 and $35 per round!

PDWs vs. Submachine Guns: Understanding the Real Difference

While PDWs and submachine guns may appear similar at first, they were developed for different purposes, which is reflected in their design and performance. Submachine guns are a proven, versatile choice for close-range combat, whereas PDWs are engineered for specific scenarios where compactness, low weight, and the ability to defeat ballistic protection are essential.

The distinction goes beyond just caliber – it’s about the intended use. This purpose is what determines when to choose one over the other, and why both categories still coexist in today’s arsenal.

For civilians, PDWs are more of a niche option, while submachine guns, particularly in the form of PCCs, offer a broader selection and greater accessibility. When making a choice, it’s important to look at both the weapon's specifications and its practical application, including the costs involved.