I have been pondering this question for a bit and being new to reloading I haven't figured it out yet. Why do you use less powder when loading a larger bullet for the same gun type. It seems you would need more powder to get the bigger bullet going out of the gun. I am sure it has something to do with the pressure to get the bullet going but I don't know what it is.

Wes

You use less powder for two reasons.

One, like you mentioned, is pressure. A heavier bullet with the same charge used in a lighter bullet will make more pressure because heavier bullets are simply harder to move. Often the pressure high higher than the maximum allowed, so you load it with less powder.

Another reason is heavier bullets can take up more space inside the case. In this situation you have a smaller charge simply because you can't fit more powder into the case.

I agree with Fumbler's comments. Yet, another approach is to go to slower burning powders, which "like" the heavier bullets to glean full benefit from their slow burning properties.