Originally Posted by
darkker
It ain't so simple.
Loose headspace may allow for the brass to grow, adding very slightly to case capacity. But that is AFTER fireforming, and the first shots to fireform will be lower power than you thought, because for the increased growth you need to rob energy to grow the case.
Increasing headspace also lengthens the distance to rifling. If you can't accommodate for that (308 with 175gr can't do it) then you have a larger jump to lands, and further bleed off g; which lowers velocity again.
If you want more velocity, you raise the pressure. 9 out of 10 times you don't need to add capacity for that, just use a more appropriate powder for the application.
... You know, physics type stuff...