The Sig Sauer MCX was born as rifle caliber replacement for the Heckler & Koch MP5SD submachine gun. Why? Well, because they could. Submachine guns developed prior to the MCX were hamstrung by technology and materials that limited their reliable operation to pistol cartridge pressure levels. But, rifle power performance in a compact form factor is what we’ve come to expect as desktop computers are replaced by phones the size of a candy bar.
Ditching the handgun cartridge and running more powerful rifle ammunition called for an updated operating system. The MP5′s roller lock, delayed blowback system is fine for pistol caliber loads, but these systems struggle to deal with the higher pressures of rifle caliber loads. While the MP5 is arguably one of the most reliable guns on the planet, you can look at the HK G3 for a hint of how the higher chamber pressure of a rifle round affect roller lock, delayed blowback systems. In a blowback system, the brass case is still filled with chamber gasses at the beginning stage of extraction. HK had to use a fluted chamber in the G3 so the brass has room to expand and contract in order to provide reliable extraction. (Even so, torn cases are not unheard of in the G3.)
In a Stoner system, the gasses are vented through the gas port and the chamber pressure is much lower by the time extraction happens. So Sig Sauer decided that the rotating, locking bolt, as we’ve become used to on the Stoner AR action, is needed to provide reliable extraction and safely contain the chamber pressures of rifle cartridges.Read More