I have this posted in the SIG forum, but thought it might be of interest in the M 16 forum.  

I have a P.A.W.S. ZX16A1 (referred to below as "M 16") and a SIG MCX 11.5" 5.56 pistol.  Parker Mountain Machine makes a trip lever that fits in the MCX upper (the upper is slotted for a trip lever as it comes from the factory) (SIG will not sell the trip lever to individuals).  The trip lever purpose is to interface between the MCX bolt carrier and the auto sear in full auto MCX lowers sold to law enforcement or military or made as post 86 samples.  Since the MCX and AR's use similar trigger/hammer groups and the takedown pin holes are in the same position for each, maybe the auto sear is in the same position.  I wondered if I could use my MCX upper on my M 16 lower and have it function in all modes.  I didn't NEED to do it...I WANTED to do it.  

PMM sold me the trip lever.  A SIG director gave me a MCX upper to AR lower adaptor (not in production yet, mine was used as a demonstrator unit at shows).  I was told adaptors will be out in 2018.  The adaptor has an internal buffer and projects above the AR lower receiver ring to hold the MCX recoil system in place.

First test was would a GI spec M 16 trigger/hammer group hit and pop a primer in the MCX upper...no, the firing pin lock prevented the firing pin from moving.  Second test was would a Geissele SSF trigger group in the M 16 lower pop a primer in the MCX upper fitted with a SIG "G" firing pin lock...success.  

OK, remove buffer tube, stock, buffer and spring, buffer detent and spring from M 16.  Attach MCX upper adaptor to M 16 lower.  Shorten the PMM supplied roll pins to .325" and install the trip lever in MCX upper.  Attach MCX upper to M 16 lower.  Attach MCX folding stock.  Test M 16 functions without ammo...pass.  Load M 16 with ammo and fire...pass!  Makes a light, handy full auto firearm.  Maybe a bit light as mag dumps sometimes pushed me back, so bursts worked better for me.  I should get better as I fire it more.  

Notes:  1)  Also put adaptor on SBR lower and shot with the MCX upper; of course since SBR lower has no autosear, it just shot semi auto.  2)  The SIG collapsing stock cannot be collapsed as it runs into the round AR lower receiver buffer tube ring which is not grooved for that stock...no big deal for me, I just use a SIG folding stock.  3) No wear noted after firing 180 rounds on the M 16 lower; everything looked same as before firing other than faint marks showing the rear of the bolt carrier touching the internal buffer piece.  4)  I put a Midwest blast diverter in place of the SIG flash hider.  

Here's a shot video and some pics

That is very cool, thanks for the work and sharing your process and the video.  Always great to see a new upper option for our M-16s!

I was told by SIG that when they start selling their Rattler upper by itself in 2018 that they will include the upper to AR lower adaptor.

Cool.  Someone made a true folding stock AR that doesn't look like warmed over blue cheese.  Good job!  And jealous.....

Dang that's some great work!! Please keep us updated on any wear points it may cause on the RR.

Fuck yea. Awesome.
I was considering filing another stamp for an ar lower so I could run a SBR MCX upper on one.

Damn.... that was one of the best M16 ideas I’ve seen in 10yrs.
Good job OP!

Apologies if I misread your first post, but what trigger did you run in your M16 lower? A Geissele SSF?

Wondering if it's somehow possible to combine a Geissele SSF with their MCX trigger for a compatible offering.

Great job OP.  I went to Parker Mountain Machine's website to check out that trip kit.  I see they also make one for the MPX.  I wonder if anyone ever figured out if an RDIAS would work in the MPX?  I have read a few threads where guys were working trying to get the Fostech Echo binary trigger to work in an MPX.  I wish I knew someone local that owned an MPX.  I would love to see how the RDIAS would line up in the sear pocket and take a few measurements.  Either the MCX or MPX would be a cool host for sure.

USGI M16 trigger group would not activate the Sig firing pin lock.  Geissele SSF would activate the Sig “G” marked firing pin lock, allowing the upper to fire.  I do not have the Sig unmarked firing pin lock to see what would activate it.  Both my Sig uppers have the “G” firing pin lock to match the Geissele triggers in my Sig lowers.

Did you have to send your uppers/BCG in or are the Sig "G" marked firing pin locks now standard on all Sig uppers/rifles?

Excellent to know that they work fine with the Geissele SSF triggers!

With this, the Q Honey Badger, the MCR Fightlite, and all the other BCM/KAC/etc. uppers available, it seems to be getting even more difficult to pick out a favorite upper combination lol.

My whole rifle was sent in during the recall.  I had a LaRue MBT in it and Sig replaced the trigger with the SSA Geissele, so they wanted the whole rifle returned.  The pistol MCX I got from an individual that told me he specified a “G” firing pin lock during the recall.  I do not know if he sent in the upper or not.

Been watching a lot more videos on the Sig MCX lately... any concerns regarding your front takedown pin hole?
On a normal AR15, the handguard secured to a barrel nut. On a Sig MCX upper, it seems the handguard is secured only by the front takedown pin. I bet I'm just being paranoid, but is there any stress or force that might accidentally be transferred to the lower there?

A DIAS will work. I've done a bunch of conversions both with a receiver conversion and a DIAS.

I would not be that concerned with that it is just a small pin than links in with the takedown pin to keep the handguard from sliding out of its rail.

Any drawings or solid models of the trip you can share?

I would like to know more about this if you don’t mind sharing.  Did the lower have to be milled to accept a DIAS or do they drop in?  Is there trigger slap with a select fire FCG in the gun?  I notice from the factory Sig puts some kind of “brace” over the disconnector to protect it.  If it is drop in and takes standard M16 FCG and the sear trip extension piece might be cool to try one at some point.

This is awesome.
I was thinking of selling my RDIAS, now I'm not so sure.

https://m.youtube.com/watch?v=XKVgczZKObI
See vid above...Sept.2015...was using a Gen 1 Sig MPX with custom trip.. Identical to the one PMM sells.  DIAS was unmodified... Upper lug has a radius on the backside of it though...few minutes with a hand file fixed that.
Lower needed no milling.
However some may argue that you can't legally use a RDIAS in an MPX as some would say it is not part of the AR family which is why I was using a post sample DIAS.

Hi Amphibian, welcome back.

Yea I have wondered if the MPX is considered at “AR” or not.  I think it is a stronger argument if an M16 FCG and RDIAS drop in.  If a lot of machine work or fitting then it’s kind of more the FNC/SCAR sear thing.  Anyway I have been messing around with the newer CMMG delayed radial blow back AR’s lately.  Glock mags are a lot cheaper anyway.  I wish I could have gotten my FM-9 belt fed to run with subsonic.  That would have been all I needed in a pistol caliber but it would not run for me.  I know some have had good results.  Not sure why mine would not.  It ran like a top with full power supersonic loads.