I am sorry this response has taken so long, but I wanted to ensure due diligence when it comes to the safety of our customers.  I personally coordinated testing and validation with CMMG's Product Design Engineers when we received several customers who were concerned with what appears to be 10mm cases bulging.  CMMG, Inc. take this very seriously as we had not experienced true case bulging in any of our durability or function testing prior to releasing this firearm.  I do want to assure you that the results of the additional testing concern that your Mk10 firearm is safe and reliable.  We removed the barrel in question off of your Mk10 and replaced with a new barrel when it arrived.  We did our testing using a CMMG owned lower and multiple barrels from different batches.  Our initial testing pre-launch focused on big name manufacturers of 10mm ammunition (Winchester, Federal, S&B, Sig, Hornady, etc.) who had offerings in the most common grains, specifically 180gr and 200gr.  With this new testing we ordered new lots of ammo from the original test ammo manufacturers and over 20 different types of ammunition from smaller manufacturers, and in offerings outside of the 180gr & 200gr.  We created the statement below based on that testing.  What you were seeing on the casing was not bulging, but case swell.  Bulging would be where the casing grows in length and the material thins in an area, which typically leads to a ruptured case.  Case swelling is what happens to all ammunition when it is fired and it forms to the chamber.  Due to the design of the 10mm casing many of the cases appear to be bulging due to the brass filling our chamber lead.  With our Radial Delayed Blowback (RDB) our bolt is locked into place when the round is fired and the delayed unlocking ensures safe operation.  When the brass is manufactured correctly and the loads are not pushes to the hot side, then this case swell into the chamber lead does not appear.

As I stated above, your firearm has been verified as safe.  Choosing a US manufacturer of ammunition in the 1,050  1,275fps will minimize the case swelling on the Mk10 10mm.  We had concerns about very hot offerings in 10mm from hand loaders and some smaller but reputable manufacturers.  The casings we were provided were a mixture of DoubleTap and Sig Sauer.  We were not able to replicate the case swelling to match the spent cases provided.  If these were reloads, that could account for the amount of swelling.  We are including some of our spent cases with the firearm.  When the casing is unchanged, but is used to contain 10mm rounds pushing 1,300fps or greater, then our firearm is still safe.  The safety comes from being built around the AR platform barrel, barrel extension, bolt, and RDB delayed unlocking.  Unfortunately, bad things happen to casings when anyone chooses to push ammunition beyond the design intent of the casing.

CMMG, Inc. Statement:

CMMG recognizes that the chamber lead and chamber in the Mk10 result in a less than 100% fully supported casing. Handguns do not have the same lead or chamber design due to the short distance and the way the round chambers with the barrels tipping/moving to assist, yet many 10mm handguns have unsupported cases also. The Mk10 chamber and lead were designed in a way to provide the most reliable feeding behavior while maintaining safe operation with properly loaded 10mm ammunition. The chamber and leads used to manufacture the Mk10 are similar to those we use on the 9mm, 40S&W, and .45ACP. With these calibers we do not see any bulging with any factory offerings of +P or below ammunition. Unfortunately, 10mm was designed in a way that pressures generated by max loads cannot be well contained in an unsupported chamber, resulting in case swelling and the inability to reload cases with certain combinations. Not every 10mm case is the same. SAAMI only defines the external dimensions of cases, which leads to varying wall thicknesses and strength. We've measured as much as a 25% difference in wall thickness between manufacturers.

CMMG has fired tens of thousands of rounds through our Mk10 platform durability testing and found no safety concerns from the case swelling. During our function and reliability testing, which tests feeding, extraction, ejection, and durability, we tested a wide variety of ammunition. We found that when the Power Factor > 230, using 180gr bullets, the greater the likelihood the casing will show visible signs of "bulging". CMMG concluded that the firearm was still safe, as it is designed to function and contain the pressures of properly loaded 10mm ammunition safely. CMMG does not recommend continuing to shoot ammunition that shows multiple signs of overpressure.  If you are seeing extreme case swelling and any other common sign of overpressure, such as ejector swipes, blown primers, split cases, etc., discontinue using that ammunition and test another load or brand. CMMG stands behinds its products and is confident in our design of safe and functional firearms. In this case, we have found the inherent design of the 10mm cartridge produces some tradeoffs when it comes to the ability to reuse and reload cases. Once again, we would like to thank you for your business and hopefully this brief explanation might help you better understand our position on this particular situation.

Thank you for your understanding and patience in this matter.  We took this claim very seriously and only had the safety of our customers at heart.  Have a terrific New Year and a blessed day.