Why do you need less powder with heavier bullets?

[NicVerAZ]

I suppose this qualifies as a noobie ballistic question, here, but when I look at recipes it seems that the heavier the bullet, the less powder I need to get to a certain power factor.

I understand that the raise in grain means lowering the velocity needed, but I would have expected a somewhat proportional need in powder weight in order to push a bigger round down a barrel.

What am I missing here?

Thanks!

[Jonathan Taliani]

The larger/heavier the bullet, the more of the case it takes up. Less space in the case results in greater pressure if powder level is the same. Accordingly, you will decrease the amount of powder.

[Skydiver]

The larger/heavier the bullet, the more of the case it takes up. Less space in the case results in greater pressure if powder level is the same. Accordingly, you will decrease the amount of powder.

Additionally, it takes a bit longer to get a heavier bullet moving and once it is moving, it is also moving slower within the barrel. This also contributes to the pressure within the barrel/cartridge/chamber.

And it's not a newbie question, it's a perfectly valid question from somebody who is curious about how things work.

Edited May 1, 2012 by Skydiver

[jester121]

I'm glad you asked it, the explanations offered seem to echo what I think I thought.

[motosapiens]

One important reason it takes less powder for a heavier bullet to make a given pf is that the pf calculation is not based in reality, and is therefore biased towards heavier bullets.

Another way of defining power factor would be based on energy, which varies with the square of the velocity. I think recoil and terminal ballistics more closely correlate with energy than with PF.

probably something that was roughly based on terminal ballistics would be the fairest and most sensible way, but the math would be harder for laypersons.

[NicVerAZ]

One important reason it takes less powder for a heavier bullet to make a given pf is that the pf calculation is not based in reality, and is therefore biased towards heavier bullets.

Another way of defining power factor would be based on energy, which varies with the square of the velocity. I think recoil and terminal ballistics more closely correlate with energy than with PF.

probably something that was roughly based on terminal ballistics would be the fairest and most sensible way, but the math would be harder for laypersons.

Very small world, Mark, very small world. We rode a couple times together.