There’s no question that compact pistols, AR-style pistols, and AR pistols chambered in different calibers have surged over the last couple of years. In late 2020, with the release of the Ruger 57, there’s also been a resurgence in the 5.7x28mm cartridge. And companies like CMMG and its Banshee Mk4 are doing it right.

The CMMG Banshee Mk4

Everyone seems to be trying to figure out how to use this miniature .223 in new and exciting ways. One such example is the CMMG Mk4 PDW pistol that is now chambered for the 5.7x28mm round. This thing is much more than just another pistol-caliber carbine, though.

5.7×28 Backstory

In the early 1980s, NATO put out a call to its ammo buddies. After seeing images of heavy body armor and new helmets on the enemies, the military alliance wanted a cartridge that offered a little more punch than the standard, run-of-the-mill 9mm that was very common in sub guns like the HK MP5, Beretta M12, and British Sterling.

This new cartridge would be used in a new class of what was termed Personal Defense Weapons. FN stepped up and sprang right into action, developing a new cartridge completely from the ground up.

The 5.7x28mm was born and introduced in 1990 as the SS90. The 23-grain plastic cored bullet boasted a muzzle velocity of 2,800 feet per second (fps). It was described as a low-recoil round with high stopping power.

FN then launched the P90 and FN Five-seveN specifically designed to chamber the new round. Later, the SS90 was discontinued in favor of the SS190, an upgraded version of the round.

Nearly a decade later, after a battery of tests conducted by NATO, FN officially received the thumbs-up from the organization, recommending it as an effective round. This nod by NATO eventually led 40 nations to employ the 5.7x28mm in law enforcement and military operations by 2006. This included the U.S. Secret Service.

Varieties of the 5.7x28mm

The 5.7 offers a .224-inch bullet in several varieties. Similar in length to the .22 WMR and despite being lumped into the small-caliber category, the 5.7x28mm packs a punch. Some loads are even capable of piercing body armor.

The high velocity alone is a sweet deal, but the low felt recoil is probably the best feature. This reduced recoil delivers faster follow-up shots and helps improve accuracy. The civilian sporting round is considerably underpowered compared to its duty variants. However, the 5.7 civilian round is still a great round for defense and just a damn fun round for plinking.

Upon searching for ammunition to feed my Banshee, I discovered this 5.7 round was not exactly easy to find. And it was not cheap when you did find it. Damn, it sure was a lot of fun, though.

The Banshee Mk4

The Banshee 300 line is CMMG’s most compact and feature-filled configuration. It comes standard with Premium Cerakote in 10 different colors. I chose the flat dark earth option because I just love the look of FDE on a small AR pistol. These are available as the pistol configuration or an SBR.

The gun is built with CMMG’s Mk4 lower receiver, single-stage trigger, and Magpul MOE pistol grip. It comes with a CMMG ambidextrous charging handle and an ambidextrous safety selector. Additionally, CMMG’s RipStock for the SBR or RipBrace for the pistol comes pre-installed.

The upper receiver is available in 5-, 8- and 16-inch configurations and features CMMG’s patented radial delayed blowback operating system.

I decided to go with the Mk4 in 5.7 with a 5-inch barrel because I love short AR pistols. And any pistol-caliber carbine that uses a standard AR lower for a pistol round is intriguing to me. The reasoning for naming it the Mk4 is that CMMG also offers the Mk57, which uses FN 5.7 magazines.

The MK4, chambered in 5.7, uses its very own proprietary magazines that hold 40 rounds. And it greatly resembles a standard AR magazine.

Both the Mk4 and Mk57 lowers use the same uppers. CMMG made this system so consumers can purchase the magazine and dedicated CMMG 5.7 upper and have them shipped to their door to build a 5.7 carbine on the lower of their choice.

CMMG wanted to offer it as a complete gun in addition to the mag/upper conversion. This brings us back to the Mk4 that I got for testing.

More Than Just a Standard AR Pistol

The CMMG Banshee Mk4 in 5.7 looks like any other AR pistol at first glance. CMMG adds a few features to these pistols that are definitely worth a mention, though.

The CMMG RML4 M-Lok handguard features a small stop at the front that I love. It provides a bit of simple added safety and comfort. The short, 5-inch threaded barrel is ready for any muzzle device or suppressor of your choosing.

CMMG adds multiple sling mounting locations, plus their oversized ambidextrous charging handle. The CMMG charging handle feels great and looks even better.

CMMG’s Micro/CQB RipBrace offers quick deployment of the RipBrace to fit each shooter’s preferred brace position. The RipBrace can be quickly deployed from a collapsed position to their personal set preference by simply ripping the brace back to the preset position.

This RipBrace deployment gives the shooter the advantage to quickly move the AR pistol from its shortest configuration to the shooter’s preferred length of pull for speedy comfort and precise accuracy. CMMG uses a shorter buffer tube, buffer, and spring. As a result, is a smaller overall length than what the comparable SB Tactical brace would offer, as well.

Shooting the Banshee Mk4

Pistol-caliber carbines are a blast to plink away with. I mounted a Sig Sauer Romeo 5XDR onto the CMMG and was ready for action. It was really a perfect red-dot for the Banshee due to the small size. And it actually matches the sleek look of the gun.

Once I sighted in the red-dot at 25 yards, I began testing this tiny beast. I was immediately able to achieve 10-shot supported sub-1-inch groups at anywhere from 10 yards to 30 yards with ease.

The CMMG Banshee MK4 ran smoothly and flawlessly through all of my testing. Not a single malfunction, no jams, and the bolt locked back every time. The Banshee 5.7 40-round magazines are fairly simple to load and remind me of a standard AR magazine.

The only problem I encountered with the proprietary magazines was the fact that the 5.7 is so tiny, and the magazine is pretty much the size of a standard PMag. This makes for a little bit of getting used to at the range while loading.

There is just a lot more magazine in your hand than the area the round is being loaded into. That makes it feel kind of strange. However, it works just fine.

I ran the CMMG Banshee indoors and only got to put 250 rounds of America Eagle through it. Specifically due to the ongoing shortage. As I always do, I ran the Banshee with no cleaning and no oiling. I also encountered no problems. This thing just wants to run, and you can’t help but let it.

The Recoil-Sensitive Mk4

There’s nearly no muzzle rise and minimal setback. Even the most recoil-sensitive shooter could run this pistol without any issues. And it’s possible to deliver fast, accurate follow-up shots.

I handed the gun to a few different people out at the range to see what others thought. The other shooters ranged in size and shooting experience and were both male and female. Everyone had the same thing to say—they all loved the gun.

One of the women had never shot an AR at all. So, she was a bit intimidated by the look of the Banshee. I walked her through the entire process to make her more comfortable. She was asking me how to buy one before the first magazine was empty.

Accuracy was excellent, shooting supported or shooting with no support while standing.

At 50 yards, the groupings opened up a little bit. My best 10-round group unsupported at 50 yards was about 3.5 inches. This is to be expected with only a 5-inch barrel, but still incredible.

On paper, I was able to achieve those sub-1-inch groups consistently shooting at up to 30 yards supported. And they only opened up a tiny bit without support. One thing I really noticed on the range was the speed of this thing. Not to mention the lack of heat it was producing.

The Upshot

The CMMG Banshee Mk4 in 5.7×28 is just a fantastic shooting and great feeling AR Pistol. CMMG really opened up the possibilities for the 5.7 round and also created a whole new market while doing it.

The fact you can purchase whatever lower you want and pair it with the CMMG upper Group Kit Mk57, which comes with three of the 40-round magazines, for only $800 and build your own custom 5.7 AR pistol is astounding.

Or you can just purchase a complete gun for around $1,500 and be ready to go out of the box. That’s a pretty sweet deal. And makes this one unique, badass option for someone looking for something a little different in the AR pistol world.

For more information, please visit CMMG.com.

CMMG Banshee Mk4 Specs

Caliber: 5.7x28mm
Barrel: 5 inches
Overall Length: 19.2 inches (collapsed)
Weight: 4.25 pounds (unloaded)
Brace: CMMG Micro/CQB RipBrace           
Sights: Picatinny rail
Action: Semi-auto
Finish: Cerakote
Capacity: 40+1
MSRP: $1,500

This article was originally published in the Combat Handguns Sept/Oct 2021 issue. Subscription is available in print and digital editions at OutdoorGroupStore.com. Or call 1-800-284-5668, or email subscriptions@athlonmediagroup.com.