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[Review] CMMG 5.7x28MM Upper: Resolute 300 MK57

It's a comeback for the FN 5.7x28mm round with CMMG's Resolute 300 MK57 upper. We hands-on test it for reliability, accuracy, fun, worth-it-nes, and more.

CMMG, known for their good looks and guns that run like scalded dogs has jumped into the 5.7x28mm conversion game. 

I recently added a complete 5.7 upper to my current CMMG Resolute 300 rifle, got a boatload of expensive ammo down the pipe, and I’m going to tell you all about it.

CMMG with optic
CMMG 5.7x28mm

By the end, you’ll hopefully know if you need to convert your favorite AR to a 5.7 and bring all the boys to the yard. 

I also did a full video review, here it is from my channel!

Table of Contents



Back in the 90s, FN created this bottleneck round as a potential replacement for the 9MM (yeah, LOL, we know) and to win a whopper of a NATO contract.

5.7 FN Round
5.7 FN Round

The 5.7×28 round was born.

at Lucky Gunner

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

But until recently it was mostly limited to the FN 5.7 pistol and the FN P90. The round has continued to gain traction, and here we are.

Bottom line, it’s a small round with big velocity. 

The Conversion

CMMG does make smaller 5.7 conversions, down to the 5” Banshee, but I wanted the big boy.

I got the Resolute 300 MK57 upper.

at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

It has a 16.1” 416SS barrel, forged receiver, CMMG handguard, and an ambi-charging handle that I genuinely like.

CMMG taking a nap
CMMG taking a nap

The brake is CMMG’s SV. Price as I tested is $925.00, which comes in a black hard coat finish. I went with the upgraded midnight bronze Cerakote finish because reasons. 

Yo, Mr. White, Science!

The heart of this upper, as well as a huge chunk of CMMG’s lineup, is the patented “radial delayed blowback” system.


After the gun goes pow, the BCG has a slight pause in travel as it rotates just a wiggle.

That delay allows a few extra nanoseconds for more gas to get down the pipe. The result is less perceived recoil and better performance when running suppressed.

End of science lesson. 

The Mailbox

What it comes with is a complete upper inside a cardboard box. Easy. Cool thing is, it can ship right to your door.

CMMG Upper

No FFL transfer needed. The not cool thing is buying ammo. More on that below. 

The Fit

The complete upper can go on any standard AR lower. Again, I paired this with my current CMMG Resolute lower and the fit was perfect.

CMMG lower side
CMMG Resolute Lower

Out of the box. Slap it on the lower. And straight to the range. 

The Optic

For this initial range review, I paired the Resolute with a Leupold Freedom RDS.

side view of the CMMG
Leupold Freedom looks good too!

This was my first go with the 1x red dot, but my initial impression is solid. It’s built like a tank.

at Amazon

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

It has a crystal clear view. And the controls are easy. So far, so good. 

If you like other stuff…check out our Best Red Dots article for more.

The Mags

CMMG has created proprietary mags for their AR conversions. They hold 40 rounds which makes me yell ‘MERICA at the range.

CMMG Magazines
40-round magazines, like the founding fathers intended!

The mags fit regular AR-15 mag wells, and I had zero issues with the mags at the range. 

The Range

After adding the optic, all I did was oil the BCG and then let ‘er rip. I used Federal American Eagle 40 grain rounds, because they are cheaper than the standard FN ammo.

CMMG inna woods
Range days are always good

I did mag dumps at short and medium distances, and I don’t have much to report.

Zero issues with feeding, ejecting, and locking back on empty. The gun runs.


The 5.7 round has a solid reputation for accuracy.  There is decades of data out there, so I was not worried about getting on target.

CMMG at the range with Johnny

I drove nails at 50 yards, literally stacking rounds, which is good news. But I am left with wondering how the longer barrel affects the velocity, compared to the 5.7 round in a pistol?

Hopefully someone in a lab coat does a test. 

The Feel

Weirdest thing about this test is that the setup is clearly an AR-15. So in my head, I was expecting an AR-15 type experience.

Nope. That radial delayed blowback just eats the little pistol round.

It is the lightest PCC I have ever worked with, which translates to an enjoyable and somewhat quiet day at the range. Nice to shoot without the neighbors cussing me because of noise. 

Why the 5.7?

With Ruger’s new release of their 5.7 pistols, and CMMG coming hard into the paint with their lineup, I think we are going to see a surge in popularity for the round.

Folks are using these for home defense SBRs and as fun guns at the range.

And it certainly is fun to mix things up. But my primary purpose is varmint hunting. RIP East Tennessee groundhogs in the coming days. 

The Good

So if you made it this far, you know this range test went fantastic. The gun runs, and spoiler alert…big thumbs up from me.

CMMG with optic

The finish is gorgeous, and the features are solid. Other than maybe adding a suppressor, there’s literally nothing else to be done to this upper.

My favorite is the CMMG “lifetime quality guarantee.” Makes me sleep better. 

The Bad

Ok, we need to talk about ammo prices. Right now FN ammo is 60-65 cents per pop.

Thankfully Federal has jumped into the game and if you dig around online, you can get down to 45-50 cents per round. But expect to pay $20 or so every time you load up a forty-round mag.

at Lucky Gunner

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons

My big hope is that the popularity of the round right now will drive those prices down a bit. 

 By the Numbers

Reliability 5/5

Sample size of one, but I couldn’t make this upper not run. 

Ergonomics 4/5

It feels like an AR. The handguard feels familiar, and the CMMG charging handle is solid.

Accuracy 4/5

It will put rounds where you tell them to go.

Customization 5/5

I guess these 5 points really go to Eugene Stoner. It’s an AR. Change anything your heart desires.

Value 5/5

CMMGs offerings don’t come at bargain-basement prices. But if you “buy once, cry once” the end result is a premier product. No upgrades needed. 

Overall 4.5/5

There’s really not much to complain about with the 5.7 upper. Solid value in a mega-fun chambering. 

at Palmetto State Armory

Prices accurate at time of writing

Prices accurate at time of writing

Available Coupons


The CMMG Resolute 300 MK57 in the glorious 5.7x28mm cartridge is reliable, stacks rounds on top of each other, and gives you the familiar AR-15 controls. It’s not a bargain but you likely won’t need any upgrades besides adding a quality optic.

CMMG kitted out and ready to go

It got my attention from day one, and overall I had a ball. The upper runs. The mags run. I’m in. 

What do you think about the 5.7x28mm cartridge? Will the new Ruger and CMMG help fuel a comeback for this little round? Let us know in the comments! If you want to look at the classic gat that started it all, take a look at our review of the FN Five-seveN!

Five-SeveN with Ammo and Mag 2
Five-SeveN with 5.7x28mm Ammo and 20 Round Magazine

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9 Leave a Reply

  • Commenter Avatar
    Rob K.

    Because of your review, I decided to order a complete Resolute 300 rifle today. Only drawback is that CMMG is back ordered for 3-5 months. Guess I will have to wait my turn. It gives me time to buy all the other toys to add to it.

    June 8, 2021 4:35 pm
  • Commenter Avatar

    Did you measure velocity differences between 5.7 shot from 5” and 16” barrel?

    March 5, 2020 7:56 pm
  • Commenter Avatar

    I think you got the scores for Accuracy and Value reversed. You say multiple times that the gun stacks rounds on top of each other. How could it be more accurate than that? Sounds like a 5/5 to me. The upper sounds like a fair value since it includes a BCG and charging handle. But the rounds are not cheap. $20 per mag dump is not a great value, so 4/5 max.

    On another note, using this round for home defense it probably not a great idea. It was made to be a good penetrator, though that depends on the bullet design. TMJ bullets at high velocity will definitely pierce standard home construction. Good penetration in a home scenario could end up with rounds in your neighbor's house that you don't want! I think some research is needed here.

    February 14, 2020 8:39 am
    • Commenter Avatar

      Caveat about the round. In many regards it's a good choice for home defense.
      It wasn't designed solely to penetrate, nor was it designed to penetrate multiple barriers.

      The initial design was to
      - Penetrate light armor at close range (Rifle length barrels will do this with almost all 5.7 ammo... Pistol length barrels will only defeat kevlar with SS198 ammo and some specialty stuff).
      - Enter/penetrate a soft medium far enough to reach vital organs.
      - Tumble in soft mediums to create a wound cavity
      - Break up after going through barriers or walls.

      February 16, 2020 6:14 am
  • Commenter Avatar

    Cool gun, but you got about twice the barrel length to the round requirement.

    I know why it is set up this way, but it's clearly best in a pistol with a 5-8" barrel length.

    I have a CMMG guard 8" AR9 and its great, these guys make good stuff.

    February 14, 2020 6:50 am
  • Commenter Avatar

    This ammo is very expensive.

    February 13, 2020 11:12 pm
  • Commenter Avatar
    John Guse

    So I have. BCA AR set up for 7.62x39. Will I need to change out the BCG for 5.7?

    February 13, 2020 3:52 pm
    • Commenter Avatar
      David, PPT Editor

      It comes with its own BCG!

      February 13, 2020 4:03 pm
    • Commenter Avatar

      Considering the 5.7 round wont function in your bca upper even if your swap the bcg since ya know its 7.62x39 not a 5.7 upper

      February 14, 2020 3:41 pm
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