Was searching old SOCNET files and came across a post i had written in January, had forgotten the whole thing:

A regular customer of ours sent us his AR15 to check WTF was wrong w it, we opened it up and checked the gas tube that it wasnt glogged cause the rifle woulndt cycle, today i took a closer look at the bolt carrier and the key, the reason had to be in there. So i was looking and checking it and then me and my collegue realized what was wrong, there was a primers metal cup, dunno what it is in english, inside the key, looked like it belonged there and we finally drilled it clear and now its A OK. Its odd that a primer comes off the case but the chances of the inside of the primer flying just right into the key is baffling. Weird Shit happens!

hmmm... heard this before on this site.

Don´t know to who this also might have happened but this is authentic, i would say BS if i would not have been present at the time.

Yes, primer cup is the correct English term or rather, American term (you know, 'mur'can). Yes, I have heard of this before and it is not totally uncommon. Usually associated with poor quality ammunition.

I've had primer cups pop loose from reloads in Federal brass and fly all over.  Never been lucky enough to have one land right in the gas key.  Good detective work, finding it.

Yes, during cleaning my AR15 after firing PMC ammo, I found a "Primer Cup" logded between the buffer and buffer detent pin (strange place), I found all of the spent casing, except the one with the missing primer cup. Heaven knows where it went to.