Daryl,
With summer hunting in many parts of Australia, the sun and extreme heat is just a day to day thing. We live with it and need to also be able to work with it.
As an example, when I guided hunters in far north Queensland, (Cape York) for wild boar (and scrub bulls), the best hunting season was at the hottest time of the year prior to the start of the wet. October was always booked well in advance.
Air temps would often be 40 C or more, and the big pigs would always be not too far away from water.
We'd use a cut down open 4WD to bash our way out to the remote swamps and water holes. This is true bush bashing with no roads.

Ammo for my scrub bull backup rifle would be in a leather belt carrier on my right side, and it would often be exposed to direct sun for lengthy periods of time.
Sometimes, due to wind direction, we would have to walk across open swamps to aproach pigs or bulls on the far side.
On some big swamps this could mean a stalk in full sun with zero shade, which could take quite a long time.
The ammo on my belt would be hot, and the barrels on my rifle are hot, even when it hasn't been fired. Put a couple of shots through the rifle in those conditions, and the barrels and chambers are then extremely hot.
I would never deliberately expose ammo to extreme heat from direct sun, but in that type of hunting you cannot spend much time worrying about how hot your barrels or cartridges are. There are too many other things taking up your time and your attention.

This is the reason that, especially for DG backup, like with wild bulls, I prefer to have a rifle firing a very low pressure cartridge like the 470 NE. The low pressure has little to do with the fact that DR's are weaker than bolt guns though. Excess pressures can also put bolt rifles out of action in certain circumstances.
It is also the reason that I shudder at the thought of someone loading a 45-70 up to produce 470 ballistics, and going after DG.

The big old English cases, when loaded to original specs but with modern powders, offer a much greater margin of safety when heat pushes pressures way up.
Over the years I've had several client hunters who have experienced problems with their (hot loads) bolt action rifles, due to cartridge pressures going way too high, because of the increased temperatures of tropical Australia.