@Kkon,
Click here:
https://www.britishairways.com/en-u...-essentials/liquids-and-restrictions#firearms
Once on that webpage scroll down till you see "Prohibited items" After you get to that point click on the "+" sign to the right of where you see "Firearms and ammunition" which is the first line item under "Prohibited items"
Please take time to read this section. You'll see that you must declare to BA at LEAST 72 hours in advance of your trip. I personally would do that as soon as you know which rifles you're taking with you. Just get that taken care of well before your trip. You'll also notice some verbage regarding a charge
"to cover the cost of our specialist firearms and ammunition handling partner."
As far as I can tell no one at BA or a rep in the UK has any intention of locking up your firearms in a safe for an indefinite period of time. From what I understand, persons who are traveling with firearms will have their gun case as well as their other checked baggage held in a "special" locked area, with access limited to only a few individuals, on the plane as well as at Heathrow airport while waiting transfer to the next connecting flight. You have a 10 hour layover, I think it's safe to say they will need to hold your stuff until the plane is at the gate and ready to be loaded.
If you think about it, wouldn't you prefer to have your luggage including your firearms in a more secure area than just sitting on a cart out in the open where any Tom, Dick or Harry would have access to your case?
Is this all a bit of a hassle in the end, yes it is. But after 4 trips to South Africa all on different airlines, including Delta who is the most hassle free when traveling with guns, I've come to the conclusion that the extra hassle is actually worth it as the airline is indeed treating my luggage and rifle case with extra care versus just another piece that could end up somewhere else than where it was headed to.
How long is your layover in Philadelphia?