Cold War Classic Cartridge in an AR: Translating 5.7x28mm into American – CMMG Banshee

The 5.7×28mm cartridge was designed for the North Atlantic Treaty Organization in the late ’80s as a replacement for the 9mm. Based on their experience in Afghanistan, Soviet forces were issuing body armor and the western alliance wanted a better cartridge with greater range, accuracy, and terminal performance so that NATO vehicle crews, operators of crew-served weapons, support personnel, special forces, and counter-terrorist groups could shoot bad guys too.

NATO called for two weapons chambered for the 5.7×28mm cartridge. FN Herstal responded by developing the FN P90 personal defense weapon (PDW) and FN Five-seven pistol. Photo courtesy of Pinterest.

FN Project 1990 PDWS and the FiveSeven Pistol

The FN P90, also known as the FN Project 1990 PDWS, is a compact select-fire personal defense weapon (PDW) with a 10.4-inch barrel. Designed in conjunction with the FN Five-seveN pistol and FN 5.7×28mm ammunition, the futuristic P90 is a compact bullpup design that has a number of unique features.

The P90 fires from a closed bolt for improved accuracy. The ergonomic thumbhole stock acts as a pistol grip and a contoured trigger guard functions as a foregrip for the support hand. An integrated tritium illuminated reflex sight provides quick target acquisition in any light.

The P90 design makes extensive use of polymer and weighs 5.6 lbs. empty, or 6.6 lbs. with a loaded 50-round magazine. The gun is handy. All controls are ambidextrous. The top-mounted magazine provides a low profile and good balance.

Over forty countries adopted the P90, including Austria, Brazil, Canada, France, Greece, India, Malaysia, and Poland. In the United States, the P90 is in service with over 200 law enforcement agencies, most notably the U.S. Secret Service. American civilians can buy a 16-inch barreled semi-automatic version called the PS90.

The civilian PS90 with a 16″ barrel. Photo courtesy of GunsAmerica.

Mexican Cartels and Terminal Ballistics

My first contact with the 5.7 was in the Customs Service. We didn’t use them, but they were being smuggled to Mexican drug cartels. They called it the “mata policia” (cop killer) for its alleged ability to penetrate armor. I say allegedly because, in the United States, 5.7 armor-piercing ammunition is restricted to military and law enforcement. The guys we caught were sending ball ammo to Mexico

We rolled up an organization that was using multiple straw purchasers to buy dozens of 5.7 guns and cases of ammunition in Texas and bringing them to San Diego to cross the border. The smugglers revealed in post-arrest interviews that the FN PS90 carbines and FN Five-seveN pistols they were carrying to Tijuana Mexico were bound for a specific cartel lieutenant.

Mexican Navy troops show captured P90. Photo by Francisco Olver courtesy of

While other cartels acquired belt-fed machineguns and rocket-propelled grenades (which will penetrate body armor), this cartel boss had the PS90’s converted to full auto and the barrels cut down by his gunsmiths. He was crazy for 5.7 and had his bodyguards carrying the P90 clones and FN Five-seveN pistols,

They told us that the FN PS90 carbines and FN Five-seven pistols were worth $5,000 in Mexico with a similar mark up on ammunition. They (and presumably their customers) were unaware that the ball ammo they provided would not penetrate Level III armor. The more you know.

5.7x28mm Ammunition

The 5.7’s flat trajectory allows for an effective range of 200 meters from a PDW and 50 meters from a pistol. The 5.7 has greater range than 9mm with nearly double the muzzle velocity and lighter recoil. You can get 50 rounds in a P90 magazine and a full-size Five-seveN pistol can hold 20 rounds with a flush magazine and 30 rounds in the extended magazine.

The FN 5.7x28mm is a rimless bottleneck design that utilizes a 5.7mm (.224 caliber) projectile. It weighs about half as much as 9x19mm parabellum and produces 30% less recoil. The FN 5.7x28mm round on the left, 5.56x45mm (.223) on the right

The original 5.7×28mm cartridge, the SS90, was introduced in 1990. It fired a 23-grain plastic-core projectile. The secret sauce was a muzzle velocity of 2,800 fps out of the P90. In 1993, the SS90 cartridge was replaced by a 2.7-mm shorter SS190 to function in the FivesSeven pistol. The SS190 uses 31 grains (producing a muzzle velocity of 2,350 fps). Fired from the FN P90, the SS190 can penetrate a Level IIIA Kevlar vest at a range of 200 meters.

What about terminal performance? According to published reports, Federal SS197SR 40gr 5.7mm VMAX makes a permanent wound cavity similar to the Winchester Ranger T-series (127gr +P+) round. While this won’t penetrate soft body armor, it will leave an ugly and permanent wound on unprotected skin.

The 5.7 has less recoil than 9mm.

CMMG Banshee

Short and handy yet long on performance.

CMMG Inc. has taken 5.7x28mm to a whole new level with the release of the 5.7 AR Conversion Magazine. If you have a 5.7 pistol, this is the perfect match. CMMG makes conversion kits and complete guns.

This is the Banshee I shot.

I recently got to shoot the Banshee in 5.7×28. I found the recoil similar to a .22 and excellent accuracy for such a small gun. Multiple shots were fast and the RMR tracked the sight picture through recoil.

A whole magazine at 25 yards produced a pleasing group on a one half scale target, the mini-me.

The Trijicon RMR was a perfect pairing, small and light. It was quickly zeroed and provided a fast precise aiming point. The intensity of the dot was quickly adjustable to provide a bright red circle easily visible even on a sunny range day.

The upper receiver has a 5-inch barrel and features CMMG’s patented Radial Delayed Blowback operating system. It ships with one 40-round 5.7 AR Conversion Magazine.

The group I was shooting with fired about 500 rounds with no malfunctions. Federal ammo performed superbly. Everyone commented on how easily the small package handled with the minimal recoil further reduced by the radial delayed blowback system.

This three-round group was fired from 50 yards standing supported. I would have shot more but I couldn’t see the holes and thought I was missing.

FN 5.7×28 is available in several commercial loads. We shot Federal total metal jackets (TMJ) with a 40-grain lead bullet, completely surrounded in copper, to produce over 320 foot-pounds of energy at 1900 fps.

CMMG’s innovative conversion magazine holds 40 rounds of 5.7x28mm.

CMMG offers a complete line of firearms in BANSHEE and RESOLUTE models that are designed for 5.7 AR Conversion Magazines. Each is built with CMMG’s Mk4 lower receiver and a dedicated 5.7x28mm Radial Delayed Blowback upper receiver. It ships with one 5.7 AR Conversion Magazine. 

CMMG’s patented Radial Delayed Blowback operating system has quickly become the gold standard for pistol caliber carbines, primarily due to the reduction in felt recoil versus direct blowback designs.

CMMG also offers complete 5.7x28mm upper receivers in BANSHEE and RESOLUTE models that are made with three different barrel lengths: 5, 8 and 16-inch. Each complete upper receiver uses CMMG’s patented Radial Delayed Blowback operating system and is compatible with 5.7 AR Conversion Magazines.

SPECS on the Banshee I used:

CALIBER: 5.7x28mm

BARREL: 5” 1:9 Twist, Medium Taper 416SS

MUZZLE: Thread Protector, Threaded ½-28

RECEIVER: Forged 7075-T6 AL M4 type upper, AR15 type lower

HANDGUARD: CMMG RML4 M-Lok handguard

FINISH: Cerakote Standard

CHARGING HANDLE: CMMG Oversized Ambidextrous


PISTOL BRACE: CMMG Micro/CQB RipBrace with 5 Position Enhanced Receiver Extension and Ambi Sling Plate

TRIGGER: CMMG Single Stage Mil-Spec Style Trigger



WEIGHT: 4lbs 4oz(unloaded)

LENGTH: ” 19.2″(brace collapsed)

CMMG 5.7x28mm AR Conversion

For gun builders, CMMG will also offer 5.7x28mm bolt and barrel kits in 5, 8, and 16-inch barrel lengths. The bolt and barrel kits will be compatible with CMMG Mk57 lower receivers, as well as Mk4/AR15 lower receivers (when used with CMMG 5.7 AR Conversion Magazines).

If you already have a lower, caliber conversion is just two pins away.

The process of converting an AR15 to shoot 5.7x28mm is simple and quick. First, attach a CMMG 5.7x28mm Radial Delayed Blowback complete upper receiver to an existing AR15 lower. Second, insert CMMG’s 5.7x28mm AR Conversion Magazine to safely feed 5.7x28mm ammunition.

CMMG recommends using a standard carbine buffer and action spring to reliably cycle the action in this configuration. The 5.7 AR Conversion Magazine also retains last round bolt hold-open functionality, allowing for faster reloads.

Product Description MSRP
Magazine 5.7 AR Conversion 40rd $39.95
Magazine 5.7 AR Conversion 40rd, 3-pack $99.95
Pistol, BANSHEE 100, Mk4 5.7x28mm $1,074.95
Pistol, BANSHEE 200, Mk4 5.7x28mm $1,249.95
Pistol, BANSHEE 300, Mk4 5.7x28mm $1,399.95
Rifle, BANSHEE 100, Mk4 5.7x28mm NFA $1,099.95
Rifle, BANSHEE 200, Mk4 5.7x28mm NFA $1,249.95
Rifle, BANSHEE 300, Mk4 5.7x28mm $1,399.95
Upper Group, BANSHEE 100, 5.7x28mm $749.95
Upper Group, BANSHEE 200, 5.7x28mm $774.95
Upper Group, BANSHEE 300, 5.7x28mm $724.95
Upper Group, RESOLUTE 100, 5.7x28mm $824.95
Upper Group, RESOLUTE 200, 5.7x28mm $874.95
Upper Group, RESOLUTE 300, 5.7x28mm $924.95

I have nothing but complements for the 5.7x28mm cartridge and the Banshee. It was easy to shoot with almost no recoil while producing impressive energy. Fit and finish was very nice and it ran very well. Everything I expect from CMMG.

The FN 5.7x28mm is an amazing round designed for a precise ballistic niche. Wither launched from a street-legal PS90, a FiveSeveN handgun or a CMMG AR, the 5.7 is soft shooting and hard-hitting. It is a joy to shoot and accurate enough for varmints or range work.

Featured photo: Mark Miller CMMG BANSHEE 300 pistol in 5.7x28mm with Trijicon RMR Pistol, Banshee™ 300

For more information visit CMMG website.

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About the author: Mark Miller is a former Customs Agent and a Green Beret who served in Afghanistan and a number of other live fire locations. A student of firearms and shooting, he is an FFL and a SOT. The guiding philosophy of his life is that terrain and situation dictate tactics and the enemy always gets a vote on any plan.

{ 5 comments… add one }
  • Ej harbet June 5, 2020, 9:39 pm

    Was not aware the vmax did similar damage to the 127gr +p+ 9mm .

  • Jon Leone June 3, 2020, 11:03 pm

    I would love to get one but these are far to expensive. Much more so than other ar uppers on the market. I see no reason for the price gap disparity. Perhaps someone can shed some light on this.

  • I Like Liberty June 1, 2020, 8:59 pm

    I do not like the 5.7 x 28 millimeter round. If others wish to buy firearms chambered in this round let them but it does not interest me. I picked common calibers like 9 millimeter Luger, .45 Auto, .40 Smith & Wesson and .308 Winchester.

    • Mark Miller June 2, 2020, 7:53 am

      I understand why you like common calibers. Other people like different things. The 5.7 is very popular in some circles and widely available. For people who just like 5.7 or already own a 5.7 handgun, the CMMG is a great option.

    • nick schaefer June 2, 2020, 9:04 am

      Seems like a fun round to shoot with near zero recoil to me. Defeating armor is a good selling point to keep in my cruiser.

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