Same parent case, same weight bullet, same powder, different results?

[jonp]

In looking up loads for my new 35 Whelen I decided to load IMR4064 under a 180gr jword to start off. The manu's site calls for 55gr to start. When looking up different calibers something struck me. The 30-06, 338-06, etc with a 180gr bullet calls for different charges of powder to reach the same velocity or there abouts sometimes over 10grs different.

So, if the parent case is identical and the weight of the projectile is the same what accounts for the difference in powder charges? Is it the neck tension caused by the larger bullets or just a function of the depth the bullet seats into the case?

[Pilgrim]

Neither...expansion ratio is most likely the variable you are looking for. As the bore gets smaller the expansion ratio goes down, so less powder produces less gas and yet equals the pressure of the larger bore. IMO...Pilgrim

[Dusty Bannister]

Bore diameter?

[Pilgrim]

Or groove diameter if you prefer. If the size of the hole increases, the expansion ratio increases, all other things being constant. This is why large capacity cases (relative to bore diameter) usually require slower powders and longer barrels. The slower powders allow the volume to increase as the powder burns (the bullet is farther down the barrel as the pressure peaks), the increase in powder results in more gas volume to fill the larger volume due to the longer barrel, and the longer barrel permits the powder gas to work on the bullet for a longer period of time which is how you get the velocity increases from magnum cases. For ex. compare the .30-06 to the .300 Weatherby. Pilgrim

[Dusty Bannister]

Better to compare the 243 Winchester, 7-08, 308 Winchester and 358 Winchester. They are basically the same case, but necked up or down for caliber.

[Moonie]

Interestingly I load 30-06 with 35gr H4895 under the 245gr Accurate Arms boolit at a velocity of 1,950fps. In my 8x57 I also use 35gr of H4985 under the Lee Karabiner at about 225gr with a velocity of 1,850fps.

[JSnover]

Is it the neck tension caused by the larger bullets or just a function of the depth the bullet seats into the case?
I wouldn't blame neck tension as much as surface area. The fatter bullets should have more of their surface in contact with the bore - more drag/friction.

[jonp]

The barrel....I was concentrating on the case and never thought about bore diameter. DOH! Sometimes you miss the forest for the trees