Kicking myself for not buying one sooner but I just bought myself a RR for my birthday. Now the wait begins..

Curious if anyone is running a MCX upper on their M16 lower. Did you need to do anything to get it to run?

I looked into it but I decided to stick with my MCR for 5.56 full-auto.

Apparently you need to get a full-auto trip bar for your upper, so the MCX BCG can actuate the autosear.  PMM sells one.  https://www.parkermountainmachine.com/product/sig-sauer-rattlervirtusmpx-full-auto-kit/

Thank you! Ordered one today and the parts to run my son’s CMMG .22 upper.

Watched some videos on the MCR I’ll have to grab one of those as well. Looks like fun.

If you read the ad:
Product Details:

1 x ​Full-auto trip bar, machined in house from 1018 steel and salt-bath nitride coated, which provides exceptional corrosion resistance and durability.
2x RATTLER/VIRTUS/MPX trip bar retaining spring pins. (Will need to be ground/machined to length)
1x RATTLER/VIRTUS/MPX full auto sear pin (Lower need to be machined to fit sear as well as sear retaining pin)


There's a real possibility that said machining will knock the value of your RR down by a four figure (i.e., $1,000 or more) amount.
Good luck.
MHO, YMMV, etc.

Is that not just referring to a virgin semi-auto MCX lower that hasn't been machined? As far as I know, the MCX just uses the same sear configuration as the M16.

From my research they are not referring to an M16 lower but a semi-auto lower that is not milled out for an auto sear. That note it probably for dealers looking to make post samples. No milling required for an M16 lower.

That's what I was thinking too. Should be plug and play after installing the trip on the bolt.

If you have a RR you don't need to machine anything on it.  Those are instructions for someone who can legally make a new MG.

You will need to shorten the pins that hold the trip bar in the upper.  Only takes a few seconds with a grinding wheel before you put them in.

Well, the Description further states:
!!!!PMM WILL NOT INCLUDE INSTRUCTIONSOR ANSWER QUESTIONS ABOUT INSTALLATION OR MACHINING ANY SELECT FIRE COMPONENTS!!!!

I'm not "Carnack the Great" and I'm not a Class III/SOT.  There's no way I'll spend $1,800 to $2,000 for and upper and another $130 for a trip kit without some sort of definitive statement from the manufacturer(s) on their website(s), not just chatter on an internet website.  And there's no way I'll grind or machine my Colt M16 lower.  The vague, indefinite language that I see leads me to believe that it is likely that some grinding and/or machining of a RR may well be required.
MHO, YMMV, etc.

Respectfully, it's really not as complicated as you're making it seem. If OP wants to pursue this, they can just as easily send the MCX upper with the PMM parts to a knowledgeable SOT to get everything running. This is no different than folks who spend several thousand dollars on a fightlite MCR upper ($6000 brand new) and then have to buy different springs, buffer weights, feed tray parts, etc to get it running properly.

There is no additional machining necessary for a registered receiver to accept an MCX upper. OP just needs to get the trip installed on the bolt and install an MCX Stock Adapter to prevent the recoil spring from exiting the upper. That's it.