I posted this in the Sig forum but someone pointed out this pertains to standard ARs as well so I’m posting it here as well.  

Just wanted to see what the level of interest would be in having some adaptors made to allow MCX uppers to fit on a standard AR lower.

I’ve called Sig repeatidly about this and am always told either they don’t have an ETA or it’ll be atleast 6 month before there’s us available.

The benefit of this peice would be to allow the use of a MCX upper on lower you’ve already SBRd and allow the use of a folding stock.

I asked Guy Votuc at Title II Arms if he’d be able to machine such a peice and he would if there was a decent market for it.

He thinks his price point would be about 195.00.

If you all could chime on your interest in this at that price point it’s be great !

i probably wouldnt use it since i dont have any MCX parts and dont really plan on getting any, but im all for having options and think having that option later down the road would be cool.

If memory serves, this kit already exists as a prototype for a drop in for some sort of SOCOM industry solicitation or RFP for some 5.56/.300 7" PDW.  The Rattler or something?

Yes Sig has been saying they will sell this piece eventually, but theres not a solid ETA on when it will be available.

Thats the reason Im seeing if people might be interested in having a third party company make it

You can drop a MCX upper onto a AR lower. Nutnfancy did it, it does batter the buffer a bit but who cares. My understanding is the adaper piece allows for the folding stock and fills in a gap on the back portion of the upper.

I'm sure it wouldn't be hard for a third party company to machine it. Its just a question of demand, which will likely be low. I'm sure someone will make it, but don't expect it to be under $100.00 unless they can sell tons of them.

The fact that sig wont just make a bunch of these is mind blowing, glad to hear someone would consider making them.

Without the MCX upper to AR lower adapter, the recoil spring assembly is held place with about an 8th of an inch of plastic contacting the upper edge of the AR lower receiver buffer tube ring (I think SIG calls it a "spring guide").  That is all that is between your face/eyes and 2 recoil springs, 2 steel recoil spring rods, and the mentioned piece of plastic.  Yes, some have fired the MCX upper on an AR lower, but without the MCX upper to AR lower adapter, there is NO WAY I'd want to fire it that way.  I've heard SIG might be making some spring guides from metal...I don't know; both mine are plastic.

The adapter projects (SIG calls that portion the "hood") above the AR lower receiver buffer tube ring and covers the rear of the channel where the MCX recoil spring assembly resides, thus fully containing the recoil spring assembly.  It also has a buffer piece inside the adaptor for the rear of the bolt carrier besides allowing for SIG stocks to be installed.  If you would like photos of same, please PM me an email address, and I'll send them.  Matter of fact, I shot my MCX pistol upper on my M16 lower this morning with the adapter in place.  Per SIG, the adapter is not in production yet.  Per SIG, both the internal buffer piece and the "hood" portion are needed to safely fire the MCX upper on an AR lower.  I am not affiliated with SIG, I only have the adapter because a SIG director gave me a unit that had been used for demonstrations with minor finish wear from having stocks attached and removed during demos.  I totally lucked out being given that gift.  I did not ask for it and felt that since it was a gift, it was inappropriate to question.

There seem to be 2 main markets for such an adapter in my opinion:  1) Owners of AR SBR's that want to put the MCX pistol uppers on their lowers and 2) M16 owners that want to ultimately shoot full auto with MCX uppers.  Also, each type of owner probably wants the easily availability of folding stocks.  

I have found 1 "flaw" in the concept.  You can't collapse the SIG collapsing stock...the end of the rods contact the round AR lower receiver buffer tube ring.  I am not going to weaken the receiver ring by cutting grooves in it to allow the stock to collapse.  No problem, I simply use their folding stocks.

Oh, one other item that may need to be addressed.  Your AR lower hammer will need to be compatible with the upper's firing pin lock.  My uppers have the "G" firing pin locks.  My SBR lower already had a Geissele SSA trigger group, so that was OK.  In the M16 lower I had to install a Geissele SSF trigger group or it wouldn't fire the MCX upper with its GI type trigger group.  

Now most shooters would have no need for an adapter, but for those who own the MCX + various AR type guns, it adds more versatility.

I would buy 3. 

I’d gladly buy one. Please have one made. I’m begging you. I have an ARAK-21 piston upper that doesn’t require a buffer tube and I’d love to be able to use MCX stocks on my standard AR lower.