One picture that I don't think I see...... How does the tip of the detent look?
Is the detent spring kinked?
Given that you have not yet tried to troubleshoot these two items, they would actually be my primary suspects.
Without the grip, how easy is it to thread the grip screw into the lower? Have you checked to see if there might be a grip screw protrusion problem into the FCG - ie. is this one of those receivers that lets that bolt thread all the way through?
Also without the grip, does the safety rotate freely, without binding?
And you're positive that you're not kinking that spring - and that it's also seating at the appropriate depth - when you're reinstalling, correct?
Given how hard it's binding (that it's physically stopping and seizing), how do your other fire control components look? The spring orientations look correct in your pictures and there doesn't seem to be anything just screaming at me, but I'm looking at a screen, here, so help me out, bruddah
- but I can't see the disconnector spring: that's in there, correct? Does the gun pass function check, prior to the safety seizing?
Some wear of the detent is not unexpected, and depending on how that mates up with the safety barrel, it could cause an issue over time. Looking at the barrel, they seemed to have a rough go at it....or maybe it's just the mileage plus being ever so slightly off with one another - tolerance stacking.
If you're not adverse to trying a cheap - but proper - fix yourself before you send it in, I think it could potentially be an easy fix, assuming that it's not the lower that's the actual problem. As
@KillerFord1977 noted, the spring and detent are both cheap: picking up a few as spares at this point would also be good. Pulling a known-working safety from another gun or buying a take-off USGI for cheap could also help in troubleshooting.
I really like that Radian safety, but I'd hate to troubleshoot by upgrading.
It's also honestly be a while since I put one in one of my ARs (
https://www.thearmorylife.com/forum/threads/lets-see-your-ar-setups.258/page-5#post-4884), and I can't remember if it came with a replacement detent and spring - or not. If it's missing the latter, you'll still want to pick up new, given our current troubleshooting matrix.