I was told that you must lock your ammunition in a case similar to a cash box for air travel.
Is this true?

It depends on the airline. All commercial airlines are bound either by the carriage of dangerous goods act or similar legislation which says ammo must be in either the manufacturers packaging or similar container that keeps every round seperate from every other round. - Hence the plastic MTM boxes are suitable, but on top of that many/most airlines have their own regulations which although not legally mandatory, the individual airline might insist on. The most common of these requirements is that ammo also be packed into a lockable and sometimes box. That particular requirement is actually ridiculous because by packing something like ammo into a metal box makes it considerably more dangerous that if it were just in a plastic or cardboard box......... nevertheless, it is a very common requirement.

The best thing you can do is check with your airline shortly before you travel for their particular requirements and get them in writing so you can prove your point if you have any problems during your travels.

Hope that helps

We have travelled from Dallas, Tx to Zim on United/SAA 5 or 6 times with the ammo carried either in the rifle case, a soft side duffel with no questions or problems

I've learned to be flexible. I pack mine in a locking plastic pistol case. It will hold 40rds of bigbore ammo, maybe 60 of 308 length in 20rd plastic boxes. I use a simple padlock. SOme airlines want this, like Lufthansa, packed separate.
I just got back form a trip thru Germany to Namibia & back. I had one 10rd wallet inside the Tuffpak & the rest of the ammo in a locked case in another checked bag, no problems.

I used a plastic case sold by Cabelas as an ammo case. It's "lockable" with flimsey toggle type luggage locks. I taped a key to the top of the case with clear plastic tape. It holds, in factory boxes, exactly 3 boxes of Federal Premium big bore ammo and two boxes of something .375 or smaller along the sides. Depending on your ammo, my weight came in just under 5.5 kilos, short a few rounds, not counting the case. They counted the case in Tanzania (the only place it was weighed), but by that time, I'd burned a few rounds and "lost" the .375's.

The rules for packing your ammunition in South Africa are very simple --

No matter how you do it, they'll ask you to do it a different way before they'll let you check your luggage. What could be simpler?

I bought a camera case made by Pelican as my Ammo box and had a TSA lock on it and put it in my checked bag.
When I departed from JNB to WDH the SAA check-in official made me remove the ammo box and check it as an additional bag. There was no charge and it arrived safely. On the return from WDH no such request and the ammo box stayed in the checked luggage. Go figure. I am glad I had bought the Pelican case even though I originally didn't think it was necessary.

I always put my ammo in a black plastic lockable ammo case that I purchased years ago from Cabela's. I never lock it, and put it in my checked rolling duffle. I have never had a problem with anyone about my ammo.

I put the ammo in a padded pistol case in the bag that does not have the rifles in it.

In addition to the ammo there is room in the pistol case for a spare scope, Leatherman, small camera and telephone.

I lock it but attach the keys with a carabiner to the handle. Locking it keeps it from being knocked open but allows easy access for inspection.

14 trips to Alaska or Africa and never a problem.