CMMG 5.7x28 conversion magazine
Courtesy CMMG
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CMMG 5.7x28 conversion magazine
Courtesy CMMG

CMMG Inc. has taken 5.7x28mm to a whole new level with the release of the 5.7 AR Conversion Magazine. This innovative new magazine holds 40 rounds of 5.7x28mm and is designed to fit and function in a mil-spec AR15 lower receiver when paired with CMMG’s 5.7x28mm Radial Delayed Blowback upper receiver.

The appeal of 5.7x28mm is that it is a small caliber with high velocity designed for a handgun and personal defense weapon (PDW). The cartridge was developed in response to NATO’s request for a replacement for the 9x19mm cartridge.

The 5.7 AR Conversion Magazine is ideal for anyone who has a mil-spec AR15 lower and wants to shoot 5.7x28mm. The 5.7 AR Conversion Magazine provides easy transition from 5.56mm to shooting 5.7x28mm while maintaining familiar controls on existing AR15 lower receivers.

CMMG 5.7x28 conversion magazine
Courtesy CMMG

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.

One the greatest benefits of using the 5.7 AR Conversion Magazine is that it allows shooters to enjoy the advantages of CMMG’s patented Radial Delayed Blowback operating system, without the need for a proprietary lower receiver. Radial Delayed Blowback has quickly become the gold standard for pistol caliber carbines, primarily due to the reduction in felt recoil versus direct blowback designs.

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 5.7x28 conversion magazine
Courtesy CMMG

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.

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’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.

CMMG is taking pre-orders now and expects to ship by mid-February.

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  1. Now this could be the ideal home-defense firearm platform (an AR-15 adjustable stock, receivers, and pistol grip with a 5.7 x 28mm barrel).

    • True, but I’ll choose to stay with established calibers and platforms. A CMMG 9mm version is preferable to me rather than this newer caliber.

      That’s not to say it won’t eventually become more common, though…for example, it took .300 BLK a while to gain steam, but now there are plenty of options and gear, and I myself have one. This 5.7x28mm might eventually pick up steam as well. Who knows…

      In the meantime, I’ve already consolidated my gun collection down to fewer, more common calibers and am hesitant to reverse that by adding an exotic.

      • How are you liking the CMMG 9? I would have to butcher the base model a bit for over here but it is an option for the wife and I.

        • Don’t have one yet…my pistol AR is .300 BLK. All my handguns are now 9mm. I do like the idea of the CMMG 9mm, though, and may opt for one some day soon.

        • Ah well if we end up going that way first I will let you know what we figure out…… Just keep in mind it’s either a mutant space gun reject or fixed mag and all the features. Either way probably going with a 9 and a 357 sig upper if everything plays nice.

        • Stay away from the cmmg 5.7 banshee, they have a serious design flaw with the magazine well, and magazine release. It looks like tbey tried to corre t that problem with this magazine. I bought one and the instant I put a magazine in it at home, I had buyer’s remorse at the wobble. It was a jam o matic, and after I received it back from cmmg it was a single shot and semiautomatic if it felt like it. I took it back to the store. I bought it as a truck gun for the new found diversity in my city, as it would have complimented my edc 5.7. There are now drive bys and pulling guns in traffic, which never used to happen, which means a gun that can go through a car door and the thug is a good thing. I know myn adrenaline rush and I can land accurate shots under pressure, so the over penetration just means I have to create the best shot, or pull my bug 9mm.

        • The CMMG 9mm is absolutely exquisite! I run it on my AR-15 lower and switch the buffer to a JP SCS with 1 tungsten weight and the yellow/red AR10 spring. Virtually no muzzle movement at all. You don’t have to switch the buffer but i do it for fine tuning the muzzle movement. Ive got about 1000 rounds through it now without a single hiccup. Highly recommend.

      • With the age of the internet ammo availability is less of a thing. 5.7×28 is out there, not crazy priced, and as we all learned at Fort Hood it is devastatingly effective.

      • Indeed, I don’t need any wild cards or exotic parts from places that might just stop selling them. I can walk into just about any LGS and even some farm and ranch stores and find what I need for common calibers.

    • Is 5.7 an ideal home defense round? As always, I guess it depends on your home and what threats you might face… but it’s not a round designed for lethality. It’s built to be better than a pistol.

      Which… okay… but I’m not sure you need to compromise on a rifle round unless you need something exceedingly compact. Like as a PDW in a vehicle or something. Otherwise a 10in 5.56 would seem to get you where you need to be without the exotic cartridge.

    • Everybody slow your roll The Banshee doesn’t use this magazine in the Mk57. Wouldn’t fit. It uses 20 round FNH pistol mag (some ship with same size ProMag I hear). Due to reliability, their words not mine, they don’t recommend anything over a 20.

      If something changed January 1 or something, I missed it.

      I am waiting for my 100 series lower (to be SBRed so fancy grip and finish not required) with a 300 series 5″ upper

  2. Like so many other guns, if I had all that money doing nothing, were I wealthy and could spend money without a care on my interests then YES, I’d buy one and a dozen magazines and 5000 rounds of crazy expensive ammo and enjoy the heck out of it.

    But being paid by the hour, I’ll stick to what I can afford. The AR-15 I already have, the pile of magazines I already have and a whole lot of cheap ammo.

    • Was thinking the same. In a pistol? O.k., high velocity in a small package – I get that. In a pdw/sub machine gun? Yeah, ok, but I can’t really get one of those. In an AR? Why, what does it do that 5.56×45 doesn’t?

      • “Why, what does it do that 5.56×45 doesn’t?”

        Playing devil’s advocate here. When you want something that small and light, 5.56 NATO isn’t the best choice for 5″ or even 8″ barrels. So a better comparison would be against 9mm. I think it’s pretty cool even if I don’t plan on getting one anytime soon, if ever. The next step in promoting this round would be more choice and better prices on ammo.

        • Looks like we had a similar thought guess I’ll need to read up on it out of a 16 inch as this would be a pain to make into a SAFE pistol.

        • At that point why not 300blk then? The platform is the same size as are the mags. Besides that 223 has way more weight for the velocity. Competing against 9mm would be more like the play Ruger made with their hand gun.

      • Keep in mind this is a wild ass guess, not targeting the 5.56 crowd at all. I am guessing it is intended to cut into the pistol caliber carbine crowd as a more powerful option than 9mm that isn’t 10 or 357 sig and is a fancy future caliber that may actually have merit (don’t know enough on 5.7). Would be interested in whether it is quieter than 5.56 and enough of a step up from 9mm to be worth a look as my wife will need something in this range soon.

      • Even less over penetration. This would be a fantastic option for HD of someone lives in an apartment complex with neighbors on 6 sides

        • 40 grain bullets seem to be the norm now. Though in the early days of 5.7mm, the bullets were much lighter than that.

        • SAFEupstateFML Federal and FNH have 40 grain FMJ and 27 grain hollow points. Then you have the Speer Gold Dot 40 grain hollow points

  3. Nope. I’ll stick with my 5.56/223. 5.7 is a boutique round. I see Jeff Quinn touting it. He gets paid to call it a “dandy” round. I’m a helluva lot more likely to get 308 for my next rifle…

  4. Everyone needs to remember 5.7 is far more optimized for short barrel applications then 5.56. In a pdw it does make sense.

    Besides options and collecting for fun is always good. Quit complaining.

    • Agreed. It’s fair to ask ‘why?” and to say “no thanks” but to reflexively state it’s unnecessary or useless seems premature. I have no obvious need for a 5.7 but am very interested in both the new Ruger 5.7 & this. Why? Because I like guns and have more money than sense!

      How does 5.7 suppress? Touching off a 5.56 (even suppressed) indoors would be pretty hellacious.

    • There was a company that used to make 5.7 AR uppers with P-90 mags, the empties dropped out of the mag well. The old ones pop up every now and then but go for over $1000. I always wanted one, when I got to the point I could afford it, they were no longer in production. They were pretty sweet from what I hear.

      • AR57 . com IIRC was their website
        I actually bought one before they were discontinued, that’s why I mentioned using P90 mags. When they start doing that again, I”ll get interested again.


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