The hammer is cocked, ready to be fired, but for some reason my AR15 won't go on safe. Everything else on the rifle works fine except for that. When I take the pistol grip off (magpul MIAD) and pull out the spring and detent, then it seems to work okay going from fire to safe and back. (It's a DPMS parts kit in a RRA reciever.)
But also, I don't think it has anything to do with the detent. Because I figure if the detent was too tight on the selector there would still be even a little play. I don't even get that. The selector will not budge barely 1/10 of a millimeter, I mean this sucker literally won't move.

It is racking my brain, I'm thinking this is one of those things where there's a simple solution, and I'm just too close to the problem. Hopefully I just need someone with a fresh perspective to clue me in.

Thanks in advance. The collective wisdom here has never failed me, and I doubt this will be the first time.

I had the same problem and it turned out to be part of the J-spring off my hammer assembly that had worked its way under the trigger.

Might want to pull your hammer/trigger parts out and check for something under there that doesnt belong.

The pointy end of the detent may be locking into the selector notch. I had the same problem with a build before. I rounded the point on the detent a little with a file, and put some CLP on it. That solved the problem for me.     YMMV.

Quoted: The pointy end of the detent may be locking into the selector notch. I had the same problem with a build before. I rounded the point on the detent a little with a file, and put some CLP on it. That solved the problem for me.     YMMV.

Yup, best fix. Other than replacing the selector....