The compact carbine is multi-caliber capable with the bolt and barrel change happening at the end-user level. The end-user can switch between 5.56 NATO, 7.62x39mm and 300 AAC.
The design uses Sig's short stroke push rod operating system. It still has a Stoner-style rotating bolt but features a gas tappet that regulates the gun's operation. In fact, the upper is fully compatible with an any AR lower.
Though Sig employees were careful not to reveal the inner working of the design, it seems obvious that the system replaces the AR's lower receiver-based buffer system with a more compact arrangement that resides completely in the upper receiver. This allows for the use of a completely collapsible or folding buttstock.
One of the challenges overcome by Sig is the ability for the bufferless operating system to run both supersonic and subsonic ammo in a suppressed or unsuppressed configuration. Sound suppressors increase gas pressure in the operating system and speed up the operating cycle of Stoner-based firearms. Removing the buffer system reduces the operating system's ability to compensate for the swing in gas pressure that occurs when switching from low pressure subsonic to hot supersonic cartridges and will present reliability challenges for the Stoner-based guns. The new, tappet-based system is more forgiving of varied gas pressure and makes it easy to run suppressed or unsuppressed sub- or supersonic ammo, with or without a can.
We'll be tracking the release of the MCX and hope to have more details on the short stroke push rod operating system as Sig shares them.
