I helped a buddy throw a lower together the other night and after building it up and mating upper to lower, I notice how loose the rear takedown pin was.

It would literally slide right out if held ejection port down.
I've never seen this before.

Is it possible the takedown pin itself was machine a fraction too small?

A new pin, detent and spring are on the way, but the detent and spring seem to be doing their job of retaining the pin itself (so it doesn't slide out) it is just loose as a goose, so to speak.

Any ideas?