The Dead Foot Arms MCS (Modified Cycle System) is a folding stock retrofit kit for AR-15s. All of the recoil parts including the spring, buffer, and bolt carrier are modified to operate in a shorter space allowing the folding mechanism to be placed behind the recoil parts. This means that AR-15s equipped with the MCS can be fired when the stock is in the folded position.
Dead Foot Arms claims the system is very reliable and even that their dual recoil spring setup eliminates bolt bounce in full auto applications. It attaches only to the lower receiver and works with any caliber/barrel length combination.
The MCS is available in kit form from Dead Foot Arms. The MCS will work with standard AR-15 stocks and Dead Foot Arms is also developing a compact, PDW style stock for use with it.