CMMG Banshee Mk57 300 Series Review

The CMMG Banshee Mk57 300 Series can be chambered in any of 10 cartridges ranging from .22LR to .458 SOCOM.

CMMG Banshee Mk57 300 Series Review

The 5.7X28mm cartridge was developed for use in the select-­fire Fabrique Nationale (FN) P90 personal defense weapon (PDW), the semiautomatic PS90 carbine and the Five-­seven pistol. Though design began in 1986, production of the P90 didn’t begin until 1990, hence the “90” in its name. The original 5.7x28mm cartridge was called the SS90, also introduced that year. It was developed as a potential replacement for the 9mm in the PDW role, offering greater range and the ability to defeat armor per a NATO requirement. A U.S. patent was awarded for the 5.7x28mm in 1991. By 2006, 40 countries had purchased firearms chambered for the 5.7x28mm for expected use among military and law enforcement professionals.

The 5.7x28mm doesn’t fit neatly in either the rifle-­ or pistol-­cartridge category. It pushes a 27-­grain bullet out of the new CMMG Banshee Mk57 at just over 2,200 feet per second (fps) to offer effective performance out to 200 yards. While it was intended for use at pistol-­caliber distances, it is loaded to rifle-chamber pressures approaching 50,000 pounds per square inch (psi), which explains its relatively high velocity from a 5-­inch barrel.

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Being blowback operated, the lightened CMMG Banshee Mk57 bolt carrier group lacks a gas key. Odd for a blowback-operated small arm, the CMMG retained the rotating bolt design to delay extraction and allow gas pressure to decrease.

The CMMG Banshee series can be chambered in any of 10 cartridges ranging from .22LR to .458 SOCOM. The platform can be configured as a pistol or as a National Firearms Act of 1934 (NFA) regulated short-­barreled rifle (SBR), offering additional concealability.

When chambered in 5.7x28mm, the action is built on a delayed, radial blowback design. This operating system combines some of the features of a traditional direct-­impingement (DI) AR-­15 with the blowback operation normally found in pistol-­caliber AR-­type firearms.


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CMMG engineered the Mk57 Banshee to operate using the 20-round FN Five-Seven pistol magazine. The magazine provided with the Mk57 is an aftermarket ProMag. Spares retail for $43.

The AR-­15 characteristic present on the Banshee is the bolt, which uses a cam pin and cam pin path cut into the bolt carrier to control rotation into and out of battery as the bolt carrier reciprocates within the upper receiver. When the bolt carrier moves forward, the bolt rotates and locks the lugs in front of the supporting lug abutments. When the group moves rearward, the cam pin forces the bolt to rotate and unlock from the barrel extension.


Absent from the bolt carrier group (BCG) is a gas key, which is usually found on DI-­type, AR-­pattern firearms. For the Banshee, a staked plug fills the gap in the bolt carrier where we’d traditionally see a gas key. This approach lets CMMG use standard AR-­15 parts and is a clever way to keep manufacturing costs down. The bolt carrier benefits from judicious machining to help lighten its weight. There are lightening cuts through the back of the carrier and some machined flats along the sides. These cuts were necessary to get the reciprocating mass low enough that the diminutive 5.7x28mm cartridge could reliably cycle the action. The reduced reciprocating mass means that the Mk57 is a very light-­recoiling firearm. The easy way to make an AR run any pistol-­caliber cartridge is to use a direct blowback system. (It’s kind of like a big rimfire.) Put enough mass behind the cartridge to keep it closed during firing, and then have it cycle back to load a new round in the chamber.

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The magazine release was designed to maintain the intuitive operation of the AR-pattern rifle, but had to accommodate the front notches of the pistol magazine.

The downside with higher-­pressure cartridges like the 5.7x28mm is that the mass becomes substantial. CMMG’s solution is both elegant and simple. By using the rotating bolt concept from the AR-­15, and some of the bolt assembly components, they’ve created a more shooter-­friendly solution. The rotating bolt allows pressure in the barrel to drop significantly before the bolt completely unlocks, so there isn’t a great need for mass to control bolt velocity. This makes the 5.7x28mm Banshee positively tame. If a new shooter is looking for something more potent than a rimfire with minimal recoil, the 5.7x28mm is an ideal choice.

Each gun ships with a 20-­round ProMag magazine, which are also used to feed the FN Five-Seven pistol. The mags are deceivingly short for the number of rounds they contain, only protruding about 2 inches from the bottom of the magazine well when seated.

The magazine posed some interesting design challenges for CMMG engineers. Each lower receiver is machined from a solid block of aluminum billet and designed to fit the Five-Seven pistol mag. This meant the Mk57 Banshee could not use a standard AR-­15 lower receiver with a plug in the magazine well to retrofit the lower. It has to engage the notches at the front of the Five-Seven mag as the pistol would. As a result, the new lower receiver has a superior magazine release than that found on the AR-­15. It’s wide and flat paddle sits flush with the side of the receiver. It offers a larger surface area for a more positive actuation.


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CMMG’s RML4 4-inch handguard supports a railed optic and M-Lok accessories. It’s also sold separately. $160

A pleasant surprise was the last round bolt hold-­open, a feature that required a fair amount of engineering to achieve. Two aluminum tabs are attached to the top of the lower receiver surrounding the front of the magazine well. The tab on the right side of the magazine well prevents over-­insertion of the magazine and ensures reliable feeding.When the magazine follower hits the tab on the left side, it pushes the bolt catch up and into the path of the bolt carrier and prevents it from traveling any further forward.

Last round hold-­open is a convenient feature at the range; it prevents that embarrassing click when you expect to hear a bang. A last-­round bolt hold-­open is essential for any activity that approaches serious use.

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The Banshee Mk3 charging handle is CMMG’s oversized ambi. It improves access under scopes. It is available seperately for $90.

The 5.7x28mm shouldn’t be a first choice for home defense, in the opinion of several Guns & Ammo editors. However, a person could do far worse. Federal Ammunition has a load that pushes Hornady’s 40-­grain Varmint Express (V-­Max) to about 2,000 fps. That bullet expands and has really good terminal performance down to about 1,800 fps. It could be an acceptable choice for engagements out to about 75 or 100 yards.


When combined with the complete lack of recoil, light weight, acceptable magazine capacity, centerfire cartridge reliability and the fact that this Banshee configuration is legally a pistol, the Mk57 makes a compelling argument.

The gun comes standard in a Cerakote finish over a hard-­coat anodized aluminum receiver. Both the upper and lower are constructed from 7075-­T6 aluminum. The Cerakote could distinguish your Banshee from others, for it’s available in 10 colors, including Bazooka Green, Burnt Bronze and Snow White, just to name a few. G&A’s test and evaluation (T&E) sample was a modest Sniper Grey.

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The Mk57 Banshee is as ambidextrous as a right-side-eject AR-­pattern rifle can be. It includes an ambi sling plate behind the receiver.

The Mk57 Banshee 300 Series pistols come complete with a short CMMG RML4 forend wearing M-­Lok compatible slots. There’s a small tab on the underside of the handguard that acts as a hand-­stop to help keep the support hand from sliding too close to the muzzle. CMMG’s new forend is short and limits support-­hand mobility, a tradeoff for gaining compactness and greater concealability.

The pistol brace found on G&A’s test pistol is a Gear Head Works Tailhook Mod 2. CMMG also offers its five-position-adjustable Micro-­CQB RipBrace that was developed in partnership with SB Tactical. The RipBrace is based on the SBA3 multi-­position brace. This brace telescopes on the proprietary five-­position receiver extension and offers up to a 12¾-­inch length of pull. Both pistol arm braces are shooter-­friendly, for sure.

While the CMMG Mk57 is a pistol, G&A evaluated accuracy at 100 yards due to the 5.7x28mm cartridge versus the standard 25-­yard test used for comparing handgun accuracy results. G&A had a large quantity of 27-­grain SS195 lead-­free (LF) jacketed hollowpoint on-­hand and used it exclusively for testing this platform.

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The SS195LF was developed for FN and features a 27-­grain projectile containing a lead-­free metallic core encased with a conventional hollowpoint jacket. The 5.7x28mm is an eminently shootable cartridge. A box of 50 rounds averages $25.

Accuracy with this round hovered around what most would consider “acceptable,” should it be compared to a pistol chambering a rifle cartridge, but good when compared to other pistol calibers.

The biggest surprise from the range was how little muzzle blast came out of the 5-­inch barrel. Although there isn’t much powder in the 5.7x28mm case, it’s still loaded to near the 50,000 psi SAAMI maximum pressure. Our expectation of noticeable muzzle blast failed to appear, leaving us pleasantly surprised.

While the 5.7x28mm ultimately never gained NATO’s support, it is a convenient cartridge when portability, lack of recoil and an effective range beyond 100 yards is a requirement. Ammo options are still somewhat limited, but loads do exist that are specific for self-­defense use or plinking at the range.

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