Cartridge size and powder burn rate.

[3006Sprng]

Quick question for a powder guru.

Smaller over-bored cartridges use slow powders (.243/25-06) as well as large cased magnums (7mm rum).

But when the bullet diameter starts increasing at a certain point, the powder burn requirement starts getting faster again.

Eg.
-300 mag, 200 grain bullet, slow powders.
-338 mag, 200 class bullet, powder requirements starts getting a tad quicker.
-or 3006 vs 338-06.

Why is this?

 

[Burn Pile]

Its due to the increased volume of the bore diameter in the barrel. As the bullet gets moving out of the case and into the barrel the volume between the chamber/case and bullet increases. To keep the chamber pressure in a good range to push the bullet a bigger bore needs a faster powder burn to make combustion gasses to keep pace with this increased volume.

243, 308, and 338 federal are all using roughly the same case but if the bullet is 6 inches down the barrel the volume of space (cubic inches, or cubic centimeters) between the bottom of the case and the base of the bullet is quite a bit different.

The powder needs to have different burn rates to fill that volume of space with combustion gasses and pressure to keep pushing on the bullet and making it go faster and to take the right amount of time for the cartridge to reach its max chamber pressure during is burn. If a powder is too fast you'll hit max chamber pressure too early and you wont have enough gas to really get the bullet moving (this is often an advantage when loading for subsonic velocities). Or you can use too much of a fast powder which gives you too much pressure and damages or destroys the gun. If the powder is too slow you wont get enough pressure because the burn/ gas generation/ pressure generation is too slow and cant keep up with the increasing volume between the case and the bullet as it travels down the barrel and your velocity will also be low. It will often burn dirty because the pressure was not high enough for long enough for a complete burn of the powder.