The frame accepts a few of the Gen 3 internals. Only the trigger bar, connector, and trigger spring. The trigger housing is their design so it will work with an FCU and so you don’t have to use a trigger housing pin but without the FCU it is needed to keep it intact. I have some that I made to work with Glock internals and one is running a GPT trigger. Can’t do that with theirs. Another is running a Timney that I managed to retrofit to their housing. The easiest way is to modify their housing and use P80 compatible one piece rear rails and modify it to fit their frame.That's the main premise of the RXM. Drop the FCU into any RXM compatible frames, just like the Sig fcu. As of the last time I looked, Ruger and Magpul only had 1 frame available. G19 size. I suppose if the sales take off like they both hope, additional models will come out, most likely a G19X or G45.
Since it accepts gen3 internals and Glock slides, a 23 or 35 are also possibilities for the public to change over to.
I was going to post a video with detailed directions on how to convert the RXM but I did not want to attract attention from some overly zealous blue state prosecutor and come after me. I also did not want to ruin it for the Ruger fans that actually like the platform and use the FCU. It is not illegal to make your own firearm and I’ve made 4 of the FrankenRugers and they work perfectly. They all seem to prefer the Magpul mags and run flawlessly with them but are finicky with some others.
Ruger is coming out with a subcompact and full size version soon. It’s already on the Magpul website but not yet available for purchase.
Here is the biggest hint I will give you and the most important piece. You can use the P80 940V2 jig for the trigger housing holes but that’s it. Use their locking block for the trigger pin hole alignment. Do not use the P80 jig for this as it won’t have the proper alignment. It will look correct but it is not and will create extra work for you.