CMMG MK57 5.7×28 8 Inch Radial Delayed Blowback AR 15 Barrel and BCG Kit

SKU: 57D18B1

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8 Inch 5.7x28mm Conversion Kit with Lightweight BCG

The 5.7x28mm round, now available to convert your AR-15 to a high-velocity, low-recoil PDW.  CMMG’s Radial-Delayed Blowback system makes this kit incredibly soft in terms of recoil, second only to .22LR.  The 8 inch barrel keeps this package highly maneuverable, while providing extra velocity for those low-weight 5.7 rounds.  Plus, the 1/2×28 threads mean you can directly thread on a 5.56 suppressor of your choosing.

Convert Your AR-15

CMMG makes caliber conversions very simple.  This kit works perfectly with a mil-spec upper and lower receiver, with no need for proprietary receivers.  CMMG also manufactures 5.7×28 magazines, in both 40 round and 10 round varieties.


  • Platform: AR-15
  • Caliber: 5.7X28mm
  • Operating System: Radial-Delayed Blowback
  • Barrel Length: 8 Inches
  • Barrel Twist Rate:  1:9
  • Barrel Threads: 1/2×28
  • Barrel Material: 4140 CM Steel
  • Barrel Finish: Salt Bath Nitride

Have Questions About this Product?

The AT3 Tactical store has a 90 day no-hassle return policy for products in good condition. You can view the full details of our return policy here.

If you encounter an issue or defect with a product after 90 days of your purchase, you can refer to the manufacturer for a warranty claim. Their policies are below.

CMMG Warranty

Seller warrants to Buyer that for a period of 6 months from the Delivery Date, all Goods will be:

  • Free from any defects in workmanship, material, and design
  • Conform to applicable specifications[, drawings, designs, samples and other requirements specified by Buyer]
  • Fit for their intended purpose and operate as intended
  • Merchantable
  • Free and clear of all liens, security interests or other encumbrances
  • Not infringe or misappropriate any third party’s patent or other intellectual property rights.

Inspection and Rejection of Nonconforming Goods

The Buyer has the right to inspect the Goods for a period of thirty (30) days after the Delivery Date. Buyer, at its sole option, may inspect all or a sample of the Goods and may reject all or any portion of the Goods if it determines the Goods are non-conforming or defective.

If Buyer rejects any portion of the Goods, Buyer has the right, effective upon written notice to Seller, to:

  • Rescind the Order in its entirety
  • Accept the Goods at a reasonably reduced price
  • Reject the Goods and require replacement of the rejected Goods.


If Buyer requires replacement of the Goods, Seller shall, at its expense, promptly/within 30 days replace the nonconforming Goods and pay for all related expenses, including, but not limited to, transportation charges for the return of the defective goods and the delivery of replacement Goods.

If Seller fails to timely deliver replacement Goods, Buyer may replace them with goods from a third party and charge Seller the cost thereof and terminate this Order for cause pursuant to Section 20.

Any inspection or other action by Buyer under this Section shall not reduce or otherwise affect Seller’s obligations under the Order, and Buyer shall have the right to conduct further inspections after Seller has carried out its remedial actions.

Disclaimer: Manufacturer’s warranty is subject to change without prior notice.