The pursuit of short operating systems for AR-15s and their derivatives has been insanely fun to watch. Options keep getting shorter and shorter, especially with the latest offerings from Battle Arms Development, Maxim Defense, and others, all using a modified base buffer system.
Upstart Dead Foot Arms (whom I did have the pleasure of meeting at SHOTShow briefly) is now in the ring with their MCS, or “Modified Cycle System”. Unlike some of the other kits, it uses a modified bolt carrier (basically cutting off the tail just past the full-auto trip sear). From there, it adds a spring in a dual recoil spring system to cut down the total cycling receiver extension to just a few inches. Thus, the kit maintains full auto compatibility of which they report their dual recoil spring eliminates any bolt bounce in full auto weapons.
The kit is available directly from Dead Foot Arms, which includes a folding stock adapter compatible with existing buffer tubes (but one may want to use the an ultra compact LWRC kit for a sane length of pull). The Adapter allows the weapon to be fired while folded through use of an angled adapter dropping the stock beneath the ejection port.
Retail is pegged at $499 which includes the bolt, springs, buffer, and folding receiver extension assembly. First shipments are expected in May of 2016 per their website. The handgun only version (not folding adapter) is set at $375.