Quote History Quoted:
Yes, Pax. That's exactly what I'm talking about.
For some reason, the buffer spring and the octogonal shaped end, in your pic, was able to go over the detent (which you've circled in yellow).
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The buffer tube (technically, receiver extension) is what holds the buffer retainer and buffer retainer spring in place. While your buffer itself may have ridden over the retainer (and if it did, it's a poor design on the buffer), but receiver extension should still be holding the retainer in place.
This is the preferred depth of the extension:
But this will work
The difference between the two is one turn of the extension into the lower.
If your receiver extension isn't holding the retainer in then it's either not screwed in far enough or is out of spec.
And, by the way, the retainer is NOT needed for proper functioning of the rifle, it just keeps the buffer from shooting out the end of the extension when you take the upper off the lower.
By any chance do you have a piston upper? It's conceivable that piston-related carrier tilt might have worn away the part of the extension that holds the retainer in place.