One thing I learned in the military is that equipment should be made so that it can’t be reassembled incorrectly. If there is a way do so, Private Snuffy will find it.
Imagine my surprise when one of my long range guests suffered a major jamming issue with his AR 10. It would fire one round, but the empty brass would jam in the left side of the chamber. Under great force, of course, requiring the release of both pins and complete removal of the lower, then much force on releasing the charging handle. After the brass was removed and the rifle reassembled, we tried loading a round and then ejecting it without firing. This was the result:
Another hard jam on the left side of the chamber, and the bullet now deeply seated. Light bulb goes off. Is it possible that the bolt could be installed 180 degrees out? Now, every M16/AR15 pattern rifle I have disassembled, the bolt cam pin hole is eccentric on one side and round on the other. It is physically impossible to insert the cam pin on the wrong side.
We found the exception – round all the way through. So 50% chance of reassembling it bass-ackwards. Huh.
KatNap Fever! 
And what is even stranger is that I have cleaned that bolt a number of times, and I have been very lucky that I reassembled it correctly all but twice!
Maybe I should buy a new LH upper!
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May the odds be ever in your favor!
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