As a medically retired U.S. Customs Service agent in South Florida during the Miami Vice era of the drug war, I can tell you that well-made pistol caliber subguns such as the Heckler & Koch MP5 still have their place in law enforcement.

The first time that I trained with the Heckler & Koch MP5 in 9mm, I became an instant fan of this well-made, accurate, reliable, and soft-shooting pistol caliber subgun.

The HK MP5, which takes a 30-round magazine, is lightweight and easy to wield, especially when used with a lightweight collapsible stock. When used with a retractable stock, the HK MP5 is compact and easy to carry in a confined area, such as during a raid on a residence or while boarding a vessel. An accessory lets you carry two magazines clipped together when operating the weapon. This makes it possible to carry at least one spare magazine and offers an easier combat reload without having to pull a spare magazine from a pocket or load-bearing vest.

When used in select-fire mode, the HK MP5 is very controllable. This makes it possible to put multiple rounds on target in a short period of time. The fact that the MP5 reliably feeds hollow-point ammunition also makes this subgun an effective platform to carry in harm's way for law enforcement officers and special ops personnel who also carry a 9mm pistol. The MP5 is available in a number of variations including the HK MP5SD Model that utilizes an integral suppressor. 

I was later issued a Walther MPK subgun (also 9mm) that was an all-steel Cold War era subgun predating the more modern HK MP5. In addition to carrying the Walther MPK while working air smuggling cases on the ground, I also carried it while working undercover. I did this in case a deal went bad, and I needed immediate access to greater firepower. Both the HK MP5 and the Walther MPK were ideal for this application because these subguns were equipped with high-capacity magazines and were very compact when stocks were folded. 

The 9mm subguns have been popular in law enforcement for many years because these firearms were basically all we had unless you wanted to carry a shotgun, rifle or carbine such as a M-16, an AR-15, Ruger Mini-14, or Colt CAR 15/M-4. If you needed to be more heavily armed while operating in a confined space, your choices were more limited than they are today. As firearms manufacturers began producing reliable M4 carbines in 5.56mm, these subguns started to fall out of favor. Even the relatively lightweight FN SCAR Heavy MK17 in 7.62mm is controllable in select-fire mode.      

Currently the weapon of choice when you need more firepower is typically an M16 rifle or the M4 carbine including select-fire models equipped with a barrel from 10.5 to 16 inches in length.

That returns us to my statement that a pistol caliber subgun can still be an effective firearm when you need more firepower. The caveat here is that it should be deployed only in the rarest of circumstances—when you need the most compact select-fire weapon possible. 

One military unit that performs dynamic tactical operations similar to some law enforcement units continues to carry pistol-caliber subguns. This unit is made up of members of the U.S. and NATO/Coalition Special Operations Command. The U.S. Navy SEALS are one example of a USSOCOM unit that effectively use an array of firearms that suit their mission template. The HK MP has been made famous by the U.S. Navy SEALS.

Another new firearm that seems to be earning an excellent reputation is the HK MP7A1, which is chambered in 4.6mm. With its compact 7-inch barrel, the MP7A1 uses a caliber that produces 50% less felt recoil than a subgun chambered in 9mm, while being accurate enough to deliver sub two-inch groups at 45 meters while being fired in semi-automatic mode. 

Even though I'm not a fan of the FN P90, this subgun arrives in 5.7x28 caliber with a 50-round magazine capacity that's capable of putting an incredible number of rounds on target in a short amount of time. The FN P90s is used today by several law enforcement and military organizations.

The popularity of the subgun in U.S. law enforcement seems to be fading, as more and more LE agencies transition to semi-automatic and select-fire M4 carbines. One factor that's contributing to the transition to M4s with different barrel lengths is that many of the older HK MP5s are wearing out. Also, the DOD is giving away warehouses full of Vietnam vintage M16s in brand new or excellent condition to interested LE agencies. 

The M16/M4 has now become the top patrol and tactical rifle/carbine in U.S. law enforcement because agencies allow sworn personnel to carry an agency issued M16/M4 variant. Tactical teams are also adopting short barreled M4s.

Because of U.S. gun laws, it's not possible for an officer to buy and carry a short-barreled subgun, unless it's a semi-automatic version with a long civilian-legal barrel. This would defeat the purpose of using such a weapon. Thus, it makes it more practical for officers to buy and carry a 5.56-caliber M4 with a 16-inch barrel and a six-position collapsible stock (also civilian legal).

Even LE agencies that have used the 9mm Colt Commando—an M4 variant chambered in 9mm—are transitioning to M4s in 5.56 because it makes sense to have one platform that can easily be kept up and running by department armorers. 

Also, 9mm is becoming less standard a caliber for duty pistols. Excluding the NYPD, law enforcement agencies have transitioned en masse to .40 S&W caliber pistols, .45 ACP, and .357 SIG. If your officers carry pistols in these calibers, it makes no sense to buy a 9mm subgun when it's just as easy to buy M4s in 5.56.

Other resources will be expended by stocking your armory with subguns.

The agency must dedicate time to train sworn personnel to use the weapon. Also, agencies must purchase spare parts and send an armorer to school to learn how to maintain that firearm. When budgets are tight, it makes more sense to use M4s and not bother with allocating money from an already strained budget just to support a few subguns. 

If your department still issues 9mm pistols and your armory has subguns in fine working order, I see no reason to stop using these firearms in certain situations. Properly trained LEOs with a reliable subgun who are posted in public view can effectively react to a potential terrorist threat. I also believe that one or two properly trained members of a backup team who provide tactical support to an undercover agent could certainly benefit by being armed with a subgun or a short-barreled M4.

If subguns work for you, then train hard and use them with tremendous effectiveness. In the end, all that really matters is that you're proficient with the firearms you're authorized to use.