A buffer that's too light on an overgassed gun will cause the bolt to outrun the mag follower, and thus no lock back.
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https://www.m4carbine.net/showthread.php?t=124949
That's my thread... but it was not the hammer pin - it was the trigger pin.
http://img19.imageshack.us/img19/9457/brokenpin4.jpg
http://img5.imageshack.us/img5/33/brokenpin3.jpg
Did it look like that ^^^^^ ?
^^^^^^^^^^^^^
Exactly like that...same look to the break and all.
Too light of a buffer can prevent lock back of the bolt. I know it may seem counter intuitive to you, but if you read the buffer threads and posts on like malfuntions, a heavier buffer solves the issue nicely.
And I believe your magic 8 ball is wrong, RA2Bach. Prior to looking for a change of buffer, a complete detail and cleaning inspection was made to preclude any other issues before screwing with things. With that said, I bet you're right it was a significant causal factor though.
The consensus on my thread was that it was a crappy pin.
Luckily it was very good CS on part of who I purchased the lower from and I had a new pin (multiple) in a few days.
The shop where I used to work had a Colt 635 on the rental wall and that joker used to break quite a few hammer pins. Is that a known issue with 9mm AR's due to the blow-back vs gas operation?