Alex had mentioned liking the "shorter" lower of the xcr.I thought that I recalled reading way back that the trigger needed to be "shorter" than a standard AR trigger to fit the lower... but the grip screw seems like a straight up mistake.
I'm not a machinist and barely got an engineering degree 20 years ago.... but it looks like approx .25" would have allowed AR trigger usage. I haven't looked in awhile but I think the centers for the pins and safety are very close to the AR layout. Xcr has a larger diameter barrel for the safety.
It seems like it would have been so easy. So many more doors opened up.
I'd drop high hundreds for a xcr lower that took AR parts with the pic rail built in for stock attachment.
Then do it again for one that to ak74 mags.