The empty space in the case adds to the pressure spike problem. In Graeme Wright's book "Shooting the British Double Rifle" he includes reference to some testing done many years ago where a tube was filled with a small amount of powder and the location/disbursion was varied. Pressure was monitored at different places along the barrel. If the powder was spread uniformly along the tube (even though there was still a large air space) the pressure was uniform throughout the tube If the powder was placed at either end, the pressure spiked at both ends while the pressure was normal in the middle.

Cordite was produced in long strands that went from case end to projectile underside, so a reduced charge didn't appear to suffer the pressure spikes.

Graeme's book includes the effects of different types of fillers and powders etc. I can't say too much without risking copyright issues. And I'm not getting a commission from Graeme for mentioning his book either.

Long throats exacerbate the ignition issues. I've had lots of drama getting reduced loads with cast projectiles to light up reliably in my 458WM (it's a CZ550 magnum). I've managed to get a few projectiles stuck in the barrel on different occassions.

Always be sceptical of any load advice or recommendations given. Publications would be more reliable source and even contacting the powder companies is worth a go.