The rise of the submachine gun – the pistol-round firing automatic carbine – began around World War I, picked up in World War II, and arguably peaked for several decades subsequent to its technological zenith in the 50s and 60s after the invention of still-prolific systems like the Heckler and Koch MP5 and the UZI. Whether through correlation or causation, it seems that the use of the 9mm SMG platform has seen an inversive decline coinciding with the emergence of abbreviated rifle-round carbines such as the 5.56x45mm Colt Commando of the sixties and seventies (an AR15/M16 platform with a 10″ or 11.5″ barrel), the AK-74U of the late seventies, and their respective, still-reigning successors, the Mk18 CQBR and the AK-10x series, for example.

While it appears that military/police use of the SMG has declined sharply over the last thirty years, there are numerous advantages of the pistol-caliber carbine (hereafter “PCC”) that the rifle-chambered PDW or carbine cannot match: For one, the PCC’s less-concussive muzzle blast has numerous benefits:

(1) Reduced report makes the PCC easier to handle in indoor settings where a muzzle blast can be blinding or deafening (even with the proper hearing protection, many rifle-cal. SBRs generate a sinus-jostling shock, especially indoors), or in any situation where you don’t want to irritate nearby shooters or generally keep the noise level down. This also comes into play if one were to use the PCC for home defense, and desired a lower-penetrating and lighter-recoiling round that was still effective at close ranges and wouldn’t create a flashbang-ish muzzle blast when fired.

(2) Relatedly, the PCC is better for suppression/silencer use than a rifle caliber. Subsonic 9mm or .45ACP ammunition is much easier to find than subsonic .223 or .308, and much more prolific and less expensive than .300BLK. Moreover, something like the AAC Evo-9 is a lot lighter than a YHM Phantom 5.56/7.62, and can be easily interchanged from pistol to PCC.

(3) In addition to suppressor commonality, ammo and sometimes magazine commonality ( such as the Kel-Tec Sub-2000 ) between a pistol and a PCC has some economic and practical/tactical benefits.

There are numerous other benefits of the PCC which, in total, lead one to wonder why these platforms are not more popular. This articulation of the benefits of the PCC, however, should not be construed as a condemnation of the rifle-caliber PDW/SBR, which has some major advantages over the PCC. (I hope to post a review of my triangle-folder M92 Yugo in the near future to explore the benefits of a rifle-caliber carbine/PDW).

If you have read this far and agree that a PCC is worth investing in, then I am pleased to acquaint you with the subject of this review: CMMG’s new Mk9 AR-style series chambered in 9mm.

Specifications: The Mk9 upper is made of forged 7075-T6 aluminum, and is chambered in 9mm, with a 9″ barrel made of nitrided 4140 chrome-moly steel in a 1:10″ twist. It is a simple, pure blowback operating system. Those who plan to use suppressors or muzzle devices on the Mk9 should be advised that the barrel is threaded in 1/2″-36 – this is the case with almost all 9mm AR-style carbines, perhaps due to the significant risk that a user might accidentally install a .223 muzzle device with 1/2″x28 threads onto a 9mm upper, which could be dangerous. Accordingly, if you plan to use a suppressor, you may have to obtain a thread adapter (I recommend a 1/2″x36 to tri-lug adapter, if you can locate it) or get a new piston/sleeve for your suppressor if it has interchangeable pistons (and be sure to install a fixed barrel adapter if your suppressor has a Nielsen device).

KeyMod is a proposed replacement for the MIL-STD-1913 interface, sometimes referred to as “Picatinny rail.” In other words, KeyMod – the product of a VLTOR/Noveske collaboration – was designed to replace the traditional Picatinny rail system that has been found on most AR systems since the late nineties. KeyMod employs a keyhole-type system on the mounting surface (such as a rail) whereby accessories such as lights and lasers can be mounted without the use of tools if they have a KeyMod nut designed to fit in these “keyholes.” The primary benefit is reduced weight through eliminating un-utilized Picatinny rail interface (which extrudes from the mounting surfaces), and a more ergonomic surface as it is ‘toothless,’ unlike Picatinny rail. Dozens of manufacturers, including CMMG, have embraced this open-source technology, and it is taking the gun universe by storm at the moment. You can find more information on KeyMod here. The Mk9 comes with the excellent free-floating RKM7 KeyMod Handguard. I got a preview of the CMMG KeyMod Handguard series in my review of the CMMG Mk3 . For those unfamiliar with KeyMod, The KeyMod slots on the RKM7 are located at the 3, 6 and 9 o’clock positions and allow users to mount compatible accessories directly to the hand guard. For accessories that are not yet compatible with the KeyMod system, CMMG offers separate 5-slot rails that attached to the hand guard. For mounting sights and optics, a 1913 Picatinny rail runs across the top of the firearm. The receiver includes a 9mm bolt (which will be required for you to shoot on your AR). The Mk9 series of lower receivers have 9mm magazine-specific mag wells that look and function perfectly, however, note that if you plan to drop the Mk9 upper on a standard 5.56mm AR lower, you will need a 9mm buffer and a 9mm magwell adapter so the 32rd. Colt-pattern 9mm magazines will fit in your lower. There are numerous options on the market, however, I personally bought the Spike’s magwell adapter specifically for use in this review. It installs in maybe sixty seconds – put it in the magwell, tighten the tension screws, and you’re all set. Better still, it allows the use of your standard magazine release unlike some magwell adapters, which are actually held in place by your lower’s magazine release, and therefore, engaging the mag release might cause the entire adapter to drop out of the well. Note that you may also need a 9mm hammer for your kit to function properly. Pistol – i.e., that price includes a full, functioning firearm/complete lower minus a buttstock. While, as I will discuss later in this review, I don’t favor the AR pistol configuration, I see the platform’s stock going up rapidly due to the recent ATF opinion letter MSRP is $1,099.95 for the Mk9 PDW– i.e., that price includes a full, functioning firearm/complete lower minus a buttstock. While, as I will discuss later in this review, I don’t favor the AR pistol configuration, I see the platform’s stock going up rapidly due to the recent ATF opinion letter wherein the ATF took the position that shouldering the SIG Pistol Brace was completely legal. Accordingly, many readers would probably consider buying the pistol, complete, and then adding the brace versus going through the requirements, expense, and wait for an SBR stamp. The pistol has a single-stage mil-spec type trigger and weighs 5.3 lbs. with an O.A.L. of 24 inches. It comes equipped with a Magpul MOE pistol grip, and an ambidextrous loop-style sling attachment point in front of the castle nut. The complete upper weighs only 3.3lbs and has an MSRP of $749.95. The pistol and upper come with the “CMMG Lifetime Quality Guarantee.”