My belt buckle has much more metal than a shell casing, and it never sets off the metal detectors at airports. Shell casings have less metal in them than the buttons on my button-fly jeans.

EDIT: I was mistaken, he did say it was an unused bullet, so it was more than a shell casing.

Yeah, that's a good point. I was thinking more along the lines of an X-ray machine (see, I don't even know enough about security processes to know if that's the right terminology) where they reveal outlines of denser objects, rather than the pass-through metal detector itself.

He actually says "bullet", I would interpret that literally - just the bullet, not a full shell and definitely not any gunpowder.

Virtually everyone I know would call a "cartridge" a "bullet", just as they might confuse "clip" and "magazine".

Educate me - what's the difference between a cartridge and a bullet? What's the difference between a clip and a magazine?

Edit: found answer to my first question below. The bullet is the inert mass. The cartridge is a bullet plus casing plus gunpowder / other energetic material.

Clip vs. magazine: http://www.thefirearmblog.com/blog/2009/02/21/clip-vs-magazi...

Huh. So a clip is a bunch of cartridges with something connecting them, whereas a magazine is an enclosed box of cartridges.

Or at least, that's what I got out of the link. That one is (still) legitimately confusing to me. It didn't directly help (I didn't notice what was strange / funny about the image until I followed through to the "Gun Zone" link).

That's possible; if so, the word "unused" threw me off. But your interpretation could also be right too.

It appears that it was in the pants, which were in the bag. Bag could be checked in and go though different check entirely.

This occurred immediately after his passport was stamped, indicating that it actually took place at immigration, not customs. (At almost all airports, you go through baggage claim after immigration (but before customs, obviously).

So the bullet was probably carried on, and thus went through security, but small items like this are missed all the time in the x-ray scanner.