bolt not locking back on last shot

[babaganoush]

I think you may have a winner, there. With little or no additional clearance, the buffer was/is likely rebounding before the follower can get into position. (Note to self - aftermarket gimmicks always profit those selling them.) I personally never let the "sproing" sound bother me. It's when the rifle doesn't make it that I know that a reload is imminent.