There's two factors that you need to consider. The first is law and the second is airline regulation.... it's very complicated but here are the bare bones to keep it as simple as possible.
The law you need to comply with is the 'Air Navigation Order (Carriage of Dangerous Goods) Act. which states your ammo must be in cases that ensure every round is kept separated from every other round OR in manufacturers packaging. Therefore the plastic MTM boxes or similar are just fine. The same order stipulates a maximum of 5 kgs of ammo per passenger and that ammo allowances cannot be combined in one box or container etc. There is no requirement for any labels of any kind.
Airline regulations (NOT law) vary on where the ammo must be packed. Some want it in the rifle case, some want it in your ordinary baggage and some want it in a separate, lockable container.
I'd recommend, you have MTM cases that will fit into your rifle case if necessary, a lockable container and space in your ordinary baggage to accommodate the lockable container if necessary.
Then all you need to do is ask the check in agent what he/she wants you to do with your ammo. If they say in the rifle case, then do that, if in the lockable container, do that and if in the ordinary bag, put the ammo in the lockable container and then into the ordinary baggage.
Other things you need to be aware of is that the aforementioned act forbids carriage of any calibre forearm or ammo that has ever been adopted by a military force anywhere in the world, ever...... the good news is that most airlines, esp American airlines very sensibly ignore that rule....... The bad news is that some airlines, esp some European airlines and even more especially BA do enforce it.
Also that the EUSSR passed a law last year that forbids the carriage of ANY ammo through Europe..... the then issued a waiver for sporting ammo but have not rescinded the original law and all it takes is an ignorant or anti hunting check in agent to know about the law but not the waiver and you're stuffed.
The good news is that there is a way over BOTH problems.
You need to go to your travel agent and have them put a comment in the comments section of your booking stating (and note the exact wording of this!)
"Pax travelling with sporting firearms and ammunition of such and such a calibre(s)"
Then ensure the travel agent has the sirline confirm in writing they are aware and happy with the situation and take several copies of that when you travel as proof.
Damn but I'm good!
