Well, the part about the 50BMG primer related to your quote about human error. Certainly was on our part ( cause we had to really give it a lick) and was something that came back to memory when I read your post.
The part about your disbelief on the 40 S&W round going off was addressed in my second part about the 9mm round being dropped and discharging.
Of course you may not agree, which I think you already have.
Bob