9mm start/max charge

[glockpro]

Hello reloader,
I have been using the Speer Reloading Manual #14 to load 9mm 147 grain JHP.
Using Unique I started with the "start" charge of 3.8 and ended up reducing it to 3.5 grs. At 3.5 grs I was able to chrono about 860 ft/sec to make a PF125+.
Ran out of 147's and now have some 115 grain TMJRN. For the lighter bullet the same book calls for more powder, a lot more. Starting at 5.6 grs. Almost 40 percent more powder.
I looked at some other cal and see the same thing.
Why does a lighter bullet use so much more powder?
Thanks,
Tim

[357magnum]

Lighter bullets are shorter in length than heavier bullets.
Heavier bullets seat deeper and would raise pressure if you used more powder, like the lighter ones.
Less case volume with heavier bullets.
Lighter bullets being shorter don't seat as deep = more case volume.
Hope this helps.

[XDRoX]

Lighter bullets are shorter in length than heavier bullets.
Heavier bullets seat deeper and would raise pressure if you used more powder, like the lighter ones.
Less case volume with heavier bullets.
Lighter bullets being shorter don't seat as deep = more case volume.
Hope this helps.

Also the heavier bullet creates more pressure as it travels down the barrel. It's harder to push. For this same reason lead also takes less powder than jacketed.

[J-cat]

A lighter bullet needs more powder to reach a velocity to create enough recoil to operate the firearm.

It does not take more powder to get 115s to 860 FPS, but that combo is too light to cycle the slide.