May 13, 2021

Spotter Up

In Depth Tactical Solutions

CMMG 5.7x28mm AR Conversion: A Cold Warrior Comes Home

4 min read

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. 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. The Trijicon RMR was a perfect pairing, small and light. It was quickly zeroed and provided a fast precise aiming point.

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.

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.

The 5.7x28mm cartridge was developed in response to NATO’s request for a replacement for the 9x19mm cartridge which could penetrate Soviet body armor and helmets. The 5.7’s flat trajectory allows for an effective range of 200 meters from a PDW and 50 meters from a pistol.
Created 1990 by Fabrique Nationale d’Herstal (FN) of Belguim, it is a rimless bottleneck design which utilizes a 5.7mm (.224”) projectile. With a total cartridge weight of roughly 93 grains, the FN 5.7x28mm weighs about half as much as 9x19mm parabellum, and produces 30% less recoil.

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.

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.

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.


SPECS on the Banshee I shot:
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
HAND GUARD: CMMG RML4 M-Lok hand guard
FINISH: Cerakote Standard
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 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.


If you already have a lower, caliber conversion is just two pins away. 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).
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.
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 and ship with one 5.7 AR Conversion Magazine.

CMMG’s 5.7 AR Conversion Magazine will be sold both as a single unit and in packs of three to reliably feed and shoot 5.7x28mm ammunition. The MSRP for the 5.7 AR Conversion Magazine is $39.95 and for the three-pack is $99.95.
Product Description
Magazine 5.7 AR Conversion 40rd
Magazine 5.7 AR Conversion 40rd, 3-pack
Pistol, BANSHEE 100, Mk4 5.7x28mm
Pistol, BANSHEE 200, Mk4 5.7x28mm
Pistol, BANSHEE 300, Mk4 5.7x28mm
Rifle, BANSHEE 100, Mk4 5.7x28mm NFA
Rifle, BANSHEE 200, Mk4 5.7x28mm NFA
Rifle, BANSHEE 300, Mk4 5.7x28mm
Upper Group, BANSHEE 100, 5.7x28mm
Upper Group, BANSHEE 200, 5.7x28mm
Upper Group, BANSHEE 300, 5.7x28mm
Upper Group, RESOLUTE 100, 5.7x28mm
Upper Group, RESOLUTE 200, 5.7x28mm
Upper Group, RESOLUTE 300, 5.7x28mm
I have nothing but complements for the 5.7 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.

CMMG Lifetime Quality Guarantee
CMMG, Inc. will guarantee its products against defects in material or workmanship. CMMG, Inc. will repair, replace or substitute part(s) (at CMMG, Inc. discretion) at no charge to the customer if a defect of material or workmanship is found. All service work must be carried out by CMMG, Inc.

About CMMG:
CMMG began in central Missouri in 2002 and quickly developed into a full-time business because of its group of knowledgeable and passionate firearms enthusiasts committed to quality and service. Its reputation was built on attention to detail, cutting edge innovation and the superior craftsmanship that comes from sourcing all their own parts. By offering high-quality AR rifles, parts and accessories, CMMG has become a leader in the AR community, while its commitment to top-quality products and professional service has never wavered.
For more information, please visit:
Featured photo: Mark Miller
BANSHEE 300 Mk4 5.7x28mm with Trijicon RMR
Pistol, Banshee™ 300, Mk4, 5.7 x 28mm | CMMG Inc.


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.