[HiBC]

What I'm going to tell you is not a perfect answer.It may help you get your head around your question.
The term that applies is "expansion ratio".
The issue is less about the case shape and more about the volume of the bore as a ratio to the volume of the combustion chamber.
As the bullet moves out of the case neck and into the bore,the volume of the bore behind the bullet is added to the volume of the combustion chamber.


As the powder burns,it produces a volume of gas. Pressure increase is about volume of gas produced vs volume available to occupy.


With a cartridge like the 30 carbine,to double the combustion chamber volume the bullet only has to move about an inch and a half.


Picture a cartridge like the 25-06. The area of the bullet base is far less,so the volume increase per inch of bullet travel is far less. But the larger case volume holds more powder,which is rapidly doubling,quadrupling,etc in volume.


So there is a race between the volume of powder gas being produced and the increase of available volume behind the bullet.


Play with the math. Use the formula "pi times radius squared= area of a circle.