Looked into the ARC, it appears to be a mash of ar15, sten, with a little mac10 thrown in for good measure.
Also found this cool little creation
https://www.thefirearmblog.com/blog/2016/04/13/frankenstem-16-p5/
Which it is pretty much what I am going after, just i want the spent casings to drop down.
I redrew the receiver section last night. I am printing part of the section right now. They are flats, they are for the mark 2 build design. At this point the 2 will be able to be printed and epoxied together, or cutout of steel or aluminum and welded together.
When the flats are done i can do some more design for tying the flats together, just think it would be easier to do my measurements on a part thats in my hands then 3d space... for flats leastways.
I will be making a fleshed out lower as well just for 3d printing (mark 1).
The mag adapter has to much wiggle on my last print, it feels great until I get the bolt contacting it and trying to run the action. thinking on wrapping it around the upper more, the metal magwell "should" be fairly breezy to make.