I’m guessing they mean like the plastic thing with the holes in it for each round. But I always figured that was to keep them from getting dinged up rather than a safety issue. Although I suppose tossing the baggage around something might poke into the primer and set one off? Would be a 1 in a billion chance though - but in an airplane...
I flew to Alaska and had slugs for my shotgun in the packaging as they told me. I forgot about it on the way back and had tossed the original boxes. So I had them wrapped up in socks/clothing and in baggies. I asked first, and they said no go - I had to get rid of them.
So I unwrapped them all, dumped them loose in the baggie and walked out to the guy helping out by the taxi stand.
“Hey - ya want 9 12-gauge Bernake slugs?” - “SURE!!”
Back then it was no big deal. But given today’s ammo situation.....!
Not just "in the airplane". If you've ever watched the luggage handling process, there are severe jolts from the time it leaves your hands until the time you pick it back up. Luggage handling is NOT gentle.