Buy it @ Rainier Arms (Click Here)

"Want to use a stock meant for a 1913/picatinny interface on any of our lower receivers, our AK receivers or our Heatseeker chassis?  Something like the Minimalist Folding Stock from Sig for the MCX/MPX?  This is the adapter you want to use!" - JSharps  

From KNS:

The AR/MCX Stock Adapter allows the user to mount a picatinny-style stock attachment (such as the SIG MPX folding stock) in place of an AR15 style buffer tube. Many firearms utilize the buffer tube attachment of the AR15 in order to mount common stocks & braces, but do not mechanically require the tube for an in-line buffer spring. For these weapons, this product offers a modular alternative for attaching stocks or other accessories.

The kit is composed of two parts and an attaching bolt. The externally-threaded adapter plug fits into the threaded buffer socket or ring. The adapter rail section firs into the slot in the adapter plug and the two are secured to each other and the firearm by a hex-socket bolt and lock washer.

Note – Standard configuration AR15 style rifles will not function using this product.

Weight 1.7 oz