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 strengths. We’ve measured as much as a 25% difference in wall thickness between manufacturers.
CMMG has fired tens of thousands of rounds through our 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 behind 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.