The CMMG Mk57: A 5.7 upper that works

CMMG continues on the 5.7×28 revival path.  Last year, we reviewed CMMG’s FiveSeven magazine compatible 5.7×28 Banshee.  Further reinforcing the 5.7×28 revival is CMMG’s new Mk57 Upper Conversion kit for standard AR lowers.  Of most import is the fact that CMMG developed a 40 round 5.7×28 magazine to ship with the Mk57 that fits into one’s AR15 lower without any modification.

CMMG Mk57 Upper Conversion

The new Mk57 conversion kits come in Banshee (pistol) or Resolute (rifle) lengths.  CMMG was kind enough to send TFB a 16″ Mk57 upper with two 40-round magazines for us to review.  Like many of their other offerings, the Mk57 comes in 10 different Cerakote finishes for an additional charge of $100 (as of writing this article), and we opted to get ours in “Bazooka Green”.

• CMMG Mk57 Magazine
• CMMG Mk57 Magazine
• CMMG Mk57 Magazine
• CMMG Mk57 Magazine

Specs, as tested:

TECH SPECS:
CALIBER: 5.7x28mm
BARREL: 16.1” 1:9 Twist, Medium Taper 416SS
MUZZLE: SV Brake, threaded 1/2-28
GAS PORT LOCATION: N/A
RECEIVER: Forged 7075-T6 AL M4 type upper
HAND GUARD: CMMG RML15 M-Lok hand guard
FINISH: Hard Coat Anodized Receivers and Hand Guard (Can be upgraded to Cerakote)
CHARGING HANDLE: CMMG Oversized Ambi
WEIGHT: 4lbs 6oz
LENGTH: 25.3

MSRP: $924.95

Initial Impressions:

CMMG ships the Mk57 upper in a cardboard box, padded by foam at both ends.  Inside the box were two 40-round Mk57 magazines and the upper.  Fit and finish seemed excellent, and the upper easily plugged into 4 different manufacturer’s AR15 lowers I had in the workshop.  The magazines also inserted positively and dropped free from all of the lowers as well.  Both Picatinny and M-LOK accessories attached flawlessly to the upper as well.

CMMG Mk57 Upper Conversion

Outfitted with an appropriate optic and mount for testing, as well as a short section of rail for mounting a bipod, the Mk57 was range ready in just a few minutes.  The 4lbs 6oz upper felt pretty standard and well balanced when mated to the two lowers taken to the range for testing.  A Larue LT-15 was chosen for having the best fitment with the Mk57, as well as an old Barrett lower that had a binary trigger fitted.

Range time

Equipped with hundreds of rounds comprised of five different loads of 5.7×28, I headed to the range with the Mk57.  Loading the mags is very easy, much easier than with the P90/PS90 platform.  Both 28gr and 40gr rounds fit just fine within the magazine body.  Inserting fully loaded magazines into the rifle with the bolt closed was rather difficult, but much easier when two rounds were downloaded.

CMMG Mk57 Upper Conversion

Initial zeroing shots were very pleasing and soft shooting.  I feel like the SV muzzle brake is gilding the lily, because the recoil of 5.7×28 is so minimal already with a 6lbs+ platform.  Both magazines started out feeding flawlessly and continued to do so with all 600 rounds fired during testing.  I did not have one magazine related malfunction during the entire course of testing.

CMMG Mk57 Upper Conversion

The new 40 rounders from CMMG have pretty much the same feel and profile as a standard AR15 magazine, and therefore are easier to reload with than smaller profile FiveSeven pistol magazines as I used in the Banshee they previously sent for review.  Also they are compatible with any standard AR mag pouch, so that is another plus.

Accuracy and reliability

Accuracy with the Mk57 was acceptable.  At 50 yards, I got comparable results to the Banshee with SS197 40gr and SS192 28gr, but worse results with SS195LF.  Groups were fired prone, with a bipod front rest and sandbag rear rest.  All results are 5 shot average, measured center-center, fired at 50y.

• American Eagle 40gr TMJ: 1.62″
• SS195LF 28gr: 1.48″
• SS198LF 28gr: 1.02″
• SS192 28gr: .88″
• SS197 40gr Hornady V-Max: .55″

The Mk57 is a very controllable platform in rapid fire.  Firing offhand at 50y, two rounds per second at 50y yielded a 2.25″ group in the head box of the target.  When mated to the lower fitted with a binary trigger, I was able to keep all rounds in the A zone of an IPSC target when firing as fast as I could at 25y.  It’s not as laser-like as a full auto P90, but it’s still a lot of fun, and just as expensive to feed!

CMMG Mk57 offhand rapid fire results

Out of 600 rounds fired with the Mk57, the only reliability issues I had was when I forgot I had a very heavy weight spring and heavy buffer in the binary lower.  The 5.7×28 just doesn’t have enough energy to cycle such a setup reliably every time.  Once I switched out the spring, I had no more issues.  The magazines never failed to feed a single round.

Overall impressions:

Objectively, the CMMG Mk57 is a reasonably accurate, reliable way to get into the 5.7×28 game via only the purchase of an upper and magazines.  The only tangible performance enhancement over a 5.56 upper is minimally softer recoil and the ability to utilize certain pistol suppressors that can handle 5.7x28mm that may be lighter than a 5.56 rated can.  Subjectively, the Mk57 is a fun and well made gateway into the 5.7x28mm renaissance that requires no paperwork or permanent modification to one’s firearm. This is greatly aided by the extremely well made magazines, which I applaud CMMG for.

Versus an AR57, the Mk57 is of higher quality in every way.  If one is looking into dipping one’s toe into the 5.7x28mm game in a large-format pistol or rifle format, give the CMMG Mk57 a serious look.

Pros:

• Compatible with any AR15 lower
• Magazines are excellent
• Reasonably accurate
• Fit and finish is excellent

Cons? (kind of a stretch but I had to come up with one):

• Doesn’t provide a performance enhancement over 5.56 in 16″ format

For more information, please visit CMMG, Inc.

Thanks to CMMG for the opportunity!