Really, it depends on the person at the counter. I called numerous times. Factory boxes. No, plastic boxes. No, only (twenty round) boxes. And, at some point, one of them said it was okay to ship the ammo in the gun case!
I went to the range and scavanged factory boxes, then chucked all of them in favor of 50 round MTM boxes, and repacked again in favor of aftermarket 20 rd. PLASTIC.
What it boils down to, is that they don't know factory from aftermarket. They all interpret the regs differently. One thing for sure, no loose cartridges stuffed in your checked baggage. And, to be safe, use plastic; they seem to prefer plastic 20 round cartons, for some reason?
One thing that can be a problem, but I have heard there is a recent change? You were limited to 5½ pounds of ammo. What the new restriction might be, I don't know? However, they never actually WEIGHED my ammo, even thought I had eleven pounds, and half of it belonged to my wife....yeah, right!
If you call ahead, to get rules, be sure and get the name of the individual. Believe me, the rules change, depending on who you are talking to.
Worst airport I have been through, by far, was Phoenix! Bad attitude toward guns, in general. Roughest treatment of guns, as well.
Good hunting. LB