I have a standard SBR lower receiver and a 11.5" MCX upper receiver. I want to marry this couple and was wondering if anyone knew if there was a certain part I could use for the back of the receiver to make this possible or should I just get an ACE Folding stock for the lower?

Sig has stated they intend to release an adapter to allow MCX uppers to work on standard AR lowers, I'm not sure if they are available yet though.

nutnfancy on youtube has a video showing a 16" MCX upper working reliably on a standard AR lower, but it slams into and mars the buffer weight.

https://www.youtube.com/watch?v=pnOMx_H1Hts

if you have a cheap buffer weight you don't care about, it looks like it will work, but personally I wouldn't just for fear of damaging the bolt carrier too.

I would wait until SIG releases the adapter. I'm wondering if it's going to make the buffer weight and spring completely unnecessary, letting you have an empty buffer tube back there. Anyone see the supposed design of the adapter?

Rock River does/is/was planning on making a receiver plug that'd allow their PDS upper to be used on an existing AR lower.  I wonder if that plate would work with the SIG.

MAC did a video showing how the MCX upper could work in a standard AR lower, but the retaining plate at the back of the bolt carrier is plastic and it sticks out halfway above the buffer tube gap; he was not confident it would stay in one piece when repeatedly slammed at the midway point, supported on bottom but not on top.

I don't have time to find the video on his account https://www.youtube.com/user/Sturmgewehre/videos

I would wait on the Sig adaptor. Nutnfancy not only gouged his buffer, but probably didn't do his buffer retaining pin hole any good. The MCX bolt carrier was hitting the AR buffer, driving it rearward. The AR buffer then returned forward to slam into the buffer retaining pin. The force of the impact gouged the buffer, and was transmitted to the lower via the retaining pin. In a standard AR, the retaining pin is only contacted by the buffer when the upper and lower are pivoted apart or separated.

Good info, Karl. Thanks!

any new update on the adapter?  I have a sig mcx upper on the way.

I talked with Sig today and he said the ar15 lower adapters should be ready around June/July.

I saw an episode of American Rifleman highliting the MCX.  Looks like the AR15 stock adaptor replaces the buffer tube completely and allows the use of their folding stock.  Neat design, I think I'm going to grab the 300 bo with adaptor when they come out.