CMMG recognizes that the chamber lead and chamber in the Mk10 result in a less than 100% fully supported casing. This is true of all semi-automatic and select fire fixed barrel firearms including, but not limited, AR15/M4/M16, AK47, and M1 Garand. 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. Comparing a handgun to a rifle or PCC is not an equivalent or fair comparison. CMMG designs the chamber and lead to be as fully supported as possible, while allowing the firearm to feed multiple brands and types of 10mm ammunition reliably. The chamber and leads used to manufacture the Mk10 are similar to those we use on the 9mm, 40S&W, and .45ACP. For those calibers we only see significant case swelling when the ammunition is loaded to what is traditionally considered beyond +P ratings. 10mm does not have +P ratings and we have sampled multiple brands and loads
Velocities from the 8” barrel range from a five shot (5) average minimum of 1,017 fps to a five (5) shot average maximum of 2,036 fps. Translating to average Power Factors ranging from 179 to 260. 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.
All casings expand when fired in any firearm. The casings can give an appearance of “bulging” when the casing expands into the chamber lead. We have found that typically when the casing diameter expansion is less than 0.0110” (~0.4315” diameter) most shooters will not see a visible bulge as it matches the rest of the case. This is the point where the case swelling is uniform and not into the chamber lead. Any case expansion greater than 0.0125” (0.4330” diameter) alerts shooters who check their brass to a potential overpressure issue. CMMG has fired tens of thousands of rounds through our durability testing and found no safety concerns from the case swell. This is because the casing is not thinner in the “bulging” area it is just forming to the chamber just like at the case opening at the other end of the chamber. CMMG did not see any of the other signs of overpressure from factory loaded ammunition with Power Factors < 220. These signs include, but are not limited to, ejector swipes, blown primers, split cases (cracks along the length or splits around the diameter), and case ruptures at the rim. Our durability loads and brands varied, but we limited the velocities to a range of 1,000 fps to 1,275 fps using 180gr projectiles. This is because this testing encompassed the most commercially available brands. Case swell was typically below 0.0085” (0.4290” diameter) with no signs of “bulging”. We considered this range to encompass standard velocity and full power loads for the 180gr FMJ/JHP ammunition.
During our function and reliability testing, which tests feeding, extraction, ejection, and durability we tested a wide variety of ammunition. This is where we recognized the need to ship the Mk10 with the H3 buffer installed, but some ammunition would require a lighter carbine buffer or the much heavier Mk10 8oz buffer (part # = 10CA976) that we developed. 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 safely. CMMG does not recommend continuing to shoot ammunition that shows multiple signs of overpressure. If you are seeing case swelling with the appearance of “bulging” and another sign of overpressure, discontinue using that ammunition and test another load or brand.