It is a crazy idea?

I have a friend who could turn out the cases with a CNC lathe and we could specify the internal volume to suit using modern powders.

The .500/645 has a water capacity of 133 grains, and pushes a 480 grain bullet 2150fps with a nominal charge of cordite.

The .458 Lott has 100 garins capacity and seems to use around 70 grains of powder in a "light" load which matches the .500/465 while the .458 Win mag is around the same charge, with a case volume of just under 90 grains.

So whould it be a good idea to make some cases with an internal volume of say 95 grains and use .458 Wim mag data as a starting point for reloading?

Is it all that dangerous/problematic reloading for these big old cases with modern powders? Do you have to use fillers?

Put your question in the double rifle thread.
Fillers ? Yes

The problem I see is that by reducing case capacity with brass and not a compressible filler you are running pressures up to modern bolt action levels well beyond the pressures the rifles were built to endure.

On the other hand, I have a first quality double rifle in 458wm that shows no signs of wear and tear from the high pressures after hundreds and hundreds of rounds. But I wonder how a rifle of lesser quality would handle it. With all jointed action guns once there is a tiny bit of play the hammering of even shotgun level pressures will loosen the action pretty quickly. Of course, a rifle or shotgun can be re-jointed to bring it back to like new tightness.

After writing the paragraph above, I wonder if the barrels of some of the older rifles, or even some of the newer rifles are up to standing modern bolt action pressure as well.

Recall that the original reason the cases were made so large was so that they could deliver their performance with lower chamber and barrel pressures, albeit with Cordite.

JPK

With the parameters you have set forth i.e., same bullet, same velocity, I would think that this is not viable. Compare the pressures of the three cartridges you mention i.e. 500/465, 458 Lott and 458 Win Mag.

From Any Shot You Want

500/465 ~36,000 cup 480gr ~2100fps
458 WM ~53,000 cup 465gr ~2200 fps
458 Lott ~53,000 cup 465gr ~2300 fps


If you merely want to reduce powder volume then there is some wiggle room dependent upon the powder selected. In an example cartridge, QuickLoad offers the following:

RL-15 69gr 1985fps ~31,000 psi 87% filled
RL-10X 58gr 1957fps ~32,000 psi 78% filled

One of the reasons for the bigger, high volume cartridge cases was to reduce pressure and or to reduce potential pressure caused by the gun/ammo baking in the sun while on safari.

Try Trail Boss...

In the previous scenario, 26 gr of IMR Trailboss fills the case about 97%, produces about 31,000 psi and yields a a velocity of 1300 fps.