CMMG’s Mk57 series is their line of 5.7x28mm firearms. Their “Banshee” model incorporates their shortest barrel in the caliber (5″), as well as a Tailhook PSB. All Mk57 models use CMMG’s Patent Pending Radial Delayed Blowback (RDB) system found in their GUARD models. The RDB blowback bolts have a head that is forced to fully rotate to unlock, causing the bolt carrier group to slow down during the firing cycle. This allows CMMG to use lighter bolts, and lighter, slower bolts mean less recoil. CMMG’s video illustrates the technology quite nicely.
BARREL: 5″ MT 4140CM SBN, 1:9 Twist, 5.7x28mm
MUZZLE: Thread Protector, Threaded ½-28
HAND GUARD: CMMG RML4
FURNITURE: Magpul MOE Pistol Grip, Tailhook MOD 2 Pistol Brace, Ambi Sling Plate, Ambi Selector, Ambi Charging Handle
RECEIVERS: Billet 7075-T6 AL Lower, Forged 7075-T6 AL Upper
TRIGGER: CMMG Single Stage Mil-Spec style trigger
MAGAZINE: ProMag 20rd 5.7×28
LENGTH: 20″ (Brace Collapsed
The CMMG Mk57 Banshee came in a Cardboard box with one magazine. Nothing too special there, but upon removing it from the rust-proofing bag it shipped in, the Burnt Bronze Cerakote immediately caught my eye. CMMG’s finishes are very nice, and this was no exception. The pistol was also light, at 4.7lbs, and the Tailhook brace was rock solid on the tube. This was my first chance to really handle a Tailhook Mod 2 brace and I must say I am very impressed at how solid of a platform it was. I consider it to be another order of quality above the SB tactical braces, which can be pretty nice in their own right.
CMMG’s single stage mil-spec trigger broke cleanly at an average of 5lbs, 6oz. Though a bit heavy, there was no grit in the pull, and the reset was on the shorter side. The unique oversized magazine release was really nice. Though right-hand only, it was very easy to manipulate and dropped the supplied FiveSeven Pro-Mag magazine free with just enough pressure. CMMG’s AR15 ambidextrous charging handle has excellent ergonomics, and functioned flawlessly. The top rail was numbered. Additionally, the key on top of the bolt was properly staked in place.
Pulling out the RDB bolt carrier, I could immediately see some differences from my other blowback AR (an old Colt SMG). Externally, it looks more like a regular AR15 bolt assembly than a blowback bolt. Internally, the major difference is an additional spring on the tail of the bolt. The RDB Bolt uses a round cam pin. There are a ton of lightening cuts in the bolt as well, and it is much lighter than other blowback AR bolts that I have encountered before.
The 5″ barrel is threaded 1/2×28″, and features a textured thread protector. A detail I really liked was the handguard that CMMG put on this gun. It has a rail at the 12 O’clock position, and slick sides everywhere else. The nicest feature of the handguard was the integrated handstop. Though small, it provides a positive ledge and reference point not to put your support hand in front of.
The CMMG Mk57 Turned in an outstanding performance at the range in terms of handling and reliability. Equipped with an Aimpoint Micro H2 red dot sight, it made for an extremely light and compact platform. Hook sling attachment points for are provided on both sides of the end plate, and I opted to use a single point bungee sling during testing.
All testing was done with both the Pro Mag and some factory 20-round FN FiveSeven magazines, along with a variety of 5.7x28mm ammo. Right off the bat, the Banshee started performing. A full mag dump as fast I could pull the trigger into an IPSC target at 25 yards resulted in all hits squarely in the center of the A zone. This was somewhat to be expected, being that the 5.7×28 cartridge produces very little recoil. (try a full auto P90 sometime, its like shooting a laser!) The 5″ barrel facilitated extremely easy handling when firing around barricades, in the shoot house, and from inside a vehicle.
All controls on the Mk57 worked perfectly. It must be noted, however, that the ProMag magazine would not drop free when fully loaded. The fit of the ProMag seemed to be tighter than the FN magazines. I’d conclude that the mag well dimensions are based upon the ProMag rather than the FN mags, which rattled around a bit and sometimes failed to lock the bolt back on empty, though they still fed well. Recoil was non-existent, allowing for incredibly fast follow-up shots due to the sights staying on target.
Reliability was excellent. Over the course of several days, I fired over 500 rounds from the Mk57 Banshee with no cleaning or oiling (other than of the barrel during accuracy testing). It ran flawlessly, and I had not one single failure to feed, fire, or eject. My only suggestion? It could benefit from some sort of slant captured recoil spring.
Accuracy and ammunition performance
The Mk57 was very accurate for a 5″ pistol. Velocities were as expected of 5.7x28mm from a 5″ barrel. All accuracy testing was done at 50 yards using a front rest, with 5 shot groups measured center-center.
- American Eagle 40gr FMJ: 1665fps
- FN SS198 27gr HP: 2240fps
- FN SS195: 2300fps
- FN SS192: 2190fps
Accuracy @ 50y:
- SS198: .57″
- SS195: .75″
- SS192: .94″
- American Eagle 40gr FMJ: 1.51″
Having a few other firearms chambered in 5.7×28, I decided to see how the CMMG Banshee Mk57 stacked up against them. The other firearms I had were a PS90 SBR (10.5″ barrel, same as the P90) and an FN FiveSeven handgun. The Banshee was almost as easy as the PS90 to control and keep on target, though the PS90 holds a slight edge in that respect. The Banshee is obviously much less concealable than the FiveSeven, though they have almost the same barrel length. I was able to obtain much better accuracy at 50y with the Banshee than the FN pistol due to its ergonomics.
The trigger pull and reset on the Banshee was batter than the P90, and about the same as the FiveSeven. It also holds an edge as far as mounting optics, ease of maintenance, and modularity. It falls short of the PS90 in one department: Barrel length. Though at a similar overall length to the P90, the Banshee has only a 5″ barrel vs the P90’s 10.5″. The FPS loss, however, is minimal.
The CMMG Banshee Mk57 is a very well made, accurate and reliable firearm. It has great fit, finish, and ergonomics, and the equipped Tailhook mod.2 is an excellent brace. Though I was somewhat leery of the ProMag it came equipped with, there was no issue with the provided magazine. Sure, the 5.7x28mm cartridge is a “niche” round, but the Banshee is an excellent choice if one should choose to shoot his cartridge. What was most exciting to me about the whole test was how reliable and smooth shooting the RDB system was. I very much look forward to see what the next calibers will be that CMMG chambers their Guard series in.
- Flawlessly reliable
- Very well made
- Somewhat short barrel for platform size
- Limited utility in this chambering
Thanks to Hughston Shooting School for range time and technical assistance.
I must thank CMMG for graciously providing the American Eagle ammo used in this test.