I think you are asking two diff questions. OAL doesn't really have anything to do with headspace, unless you load to touch the rifling. Headspace is a function of the cases fit in the chamber.
When you load longer you do increase volume a tiny bit, but pushing the bullet closer to the rifling increases pressure, so adding powder may very well put you over pressure. It's opp with handgun rounds.

__________________
NRA Cert. Inst. IDPA CSO