Here's the latest version of the lower in CAD. I've added the clearance for the Upper's bridge support and widened the space where the RA rollmark goes on the Lower; the latter has resolved the issue where the space for the mag release arm was disproportionally short.
Of particular note is that I've shifted the entire FCG .050" forward; this seemed to resolve the issue of the safety detent being too far forward (the error suggested that the safety hole was too far back) and kept the hammer pin hole centered on the front lobe. This means that the hammer now .050" forward, but it may have needed that distance to travel far enough to strike the firing pin.
It looks like at this stage, work for this lower will involve a lot of guessing and checking. It's a good thing plastic is cheap. I have to be sure to adjust my print settings to make a more durable fit test.
Seeing Jordan636's XCR with the Zhukov stock has led me to believe that some people might need the slightly longer rear picatinny rail that the Lucky Irishman adapter provides. I may try the alternative rail design when the next print gets made and see how that handles.
Small update, I've expanded the rear rail to match more like Jordan's old rifle. Should be helpful for people who need the extra length. I don't think it looks too bad either, it's a compromise between the really long rail I had in the beginning and the shortened rail that didn't appeal to me quite as much.
While I was at it, I made a model of the bolt hold open too. Its tiny details might be off but the main details like the length and screw thread location are caliper-accurate. I think it's good enough to make a replica part if necessary.
Making a model of the BHO might be necessary because when we extend the lower to fit the AR-15 FCG, we might need a longer version of the BHO to take its place. I will try to use the original BHO and use a longer screw or plastic extender to make up the quarter inch length difference before I do that however. I think doing that will still make the BHO sturdy enough for the job.
At this point I need to print another one out and do more guessing and checking. But I think I really got it close this time: cosmetically it's mostly there (and all of the production lowers I've seen seem to have some variance to them anyway) and it's only a matter of getting these hole locations right before we have a functioning receiver. The magwell was a little too tight for magazines in the first print, but it could have been due to plastic deformation that was the cause.
Recent events with AWB 2022 being passed narrowly in the US House has been a cause for concern for me, since it (in Section 2, item (40), sub-item (M)) prohibits the manufacture of receivers like this. While whether it can be passed as law is another kind of discussion, it does hold my feet to the fire to getting this thing built. At this rate, I can have both designs finalized by September and something built in metal by October or November. If I don't get a CNC machine for myself by then, there are several FFL machine shops around the nation that I contract to mill this thing out; even if it costs me $600 for one of these it'll be worth it. That should beat out legislation in the chance AWB '22 gets ratified by the end of this year. Being able to produce this thing in numbers for other users, however, gets less predictable even if I can file for an FFL form 7 in time. Even if the window for major production gets slammed shut, the fact that this data exists keeps hope alive. Either way, I need to pick it up a bit to make my target goal; August came by really quick this year, I swore February was last month...