Why do we use more powder with lighter bullets?

[0to60]

...and less powder with heavier bullets?

I can think of two possible reasons for this:

1) A lighter bullet takes less pressure to start moving, and once it does the "combustion area" becomes larger, thus reducing pressure. By adding powder, we can keep burning into this expanding space, increasing the amount of energy imparted to the bullet.

2) A heavier bullet is longer, and thus seats deeper into the case, raising pressure. To compensate, less powder is used.

Am I close?

 

[rcmodel]

Sort of close.

Inertia, and bore friction are both very important reasons.


Try hitting a Soccer ball and a Bowling ball with a doubled up fist as hard as you can, and it will quickly become clear to you!

You can hit the lighter soccer ball harder without breaking your fist.
And it will go faster then the Bowling ball if you hit it harder.

rc

 

[Sam1911]

Pressures. As rcmodel says, that heavy bullet has a lot of inertia, and more friction to overcome before it gets moving.

Fire off a max charge appropriate for a light bullet under a heavy bullet and the pressure will go way above what the barrel should have to handle before that bullet is started moving, and will continue too high until the bullet is out of the bore.