Maybe not exactly the answer to your question, but according to this site (see Section XIV, Chapter 71, HS code 7108131000), Russia imposes a 20% customs duty on the importation of gold bars.
So unless you have 5.3 million rubles on hand to pay the customs duty (or can break off a fifth of the bar), expect the customs officers to hold on to your gold bar until you come up with the money.
Be sure to get a receipt!
travel.stackexchange.com/users/1362/Nate Eldredge
It's allowed, I have personally seen a passenger who booked a seat for a bag.
I then asked the passenger and he said it was gold.
I was an operating flight attendant.
I also do not recall any rules regarding prohibiting gold onboard, (from an aviation point of view) unless it was in the shape of a knife I assume.
Just check with the airline you are flying with in case they have specific rules regarding that (I don't think they would).
Finally, make sure about the customs regulations in the destination country, that's what I would worry about.
travel.stackexchange.com/users/10051/Nean Der Thal
Anecdotally, you can buy gold at a vending machine in Abu Dhabi airport , in the outgoing terminal.
Whilst
policies at destinations will vary of course, it's highly unlikely to be widely illegal whilst being sold at an airport.
Gold ATMs are available across the world - not sure I would have believed it if I hadn't seen it myself while transiting through Abu Dhabi!
:)
travel.stackexchange.com/users/22821/StackExchange What The Heck
You are carrying a half-million dollars in gold on your person and you are flying into... Russia?
Keep a sharp eye out.
As Nate points out, you may have to pay an import duty in Russia, but apparently there are no restrictions on the British end.
In the US, you would have to declare it.
Last year, I got into a nasty run-in with ICE who were convinced I was exporting currency.
My research corresponds with yours: nothing about carrying metal on the plane.
I imagine getting through the security checkpoint is going to be a hassle though.
Gold is pretty impervious to x-rays.
travel.stackexchange.com/users/405/Malvolio
Not exactly your destination, but Werner Rydl was arrested in Brazil when he was carrying a bar of gold .
Also others have been arrested when they were unable to promptly provide the origin of the gold they were carrying.
Since you said in some comment that you will just transport the gold but you're not the owner of it...
You'd be a mule , or, if you were in Brazil, mula .
travel.stackexchange.com/users/311/woliveirajr
Can I fly with a  Good Delivery  gold bar? I'll have the certificate that I have been given with it.  I have looked on Google and found nothing that suggests it's a prohibited item on an aircraft. It's below the weight I am allowed to carry in person. I'll be taking it to Moscow, flying from Heathrow with BA.
Can I fly with a gold bar?
travel.stackexchange.com
<air-travel><legal><hand-luggage>
travel.stackexchange.com/questions/50882
travel.stackexchange.com/users/19359/Ulkoma
You may have to pay an import duty if travelling to Russia, but apparently there are no restrictions on the British end. Check the customs regulations in the destination country to be sure. Although there do not appear to be airline restrictions preventing passengers transporting gold, it is considered a good idea to check with the airline before travelling. Leaving the UK with gold also does not appear to be a problem, however, Russia imposes a 20% import duty on gold.
There may be customs restrictions on importing gold into the destination country. In Russia the tax is 20%. There are no restrictions on leaving the UK with gold. Transporting gold does not appear to be a problem with airlines, but better to check before travelling.