Looks like should have posted this here instead of in discussion --my bad.
Here's the story. I recently changed out the free float handguard and stock (switched to a CTR from a VLTOR ) and could not get the bcg to strip off a round on either a PMAG or metal mag today at the range.
The bcg will not stay back on an empty mag when pulling back on the charging handle and pushing in on the release.
I recently replaced the buffer retainer (and spring) and checked to make sure just the lip was being pushed down by the buffer tube. I lubed the bcg and release (they weren't very dirty) and still cannot get the bcg to lock back. When I release the charging handle on an empty mag the bcg moves forward but does not go all the way -- there is about a 3/4 inch gap. When I pull the trigger the bcg moves forward but still has a 1/2 gap.
I appreciate the help and advice!
maybe your buffer retaining pin popped out. it needs more than just the lip of the receiver extension to hold it in place properly, the end of the extension should be well over the edge of the pin.