gnoahh, we are on the way to changing from a factory hull dimension to a custom fit for a chamber that probably is oversize, but may not be. I've never made a practice of trying to memorize SAAMI drawings but I do recall that they include a tolerance of some amount for size. Seems like they vary from cartridge to cartridge as it won't make a lot of sense to have the same size tolerance for a 25 ACP as a 460 Weatherby. I haven't found a SAAMI drawing of chamber size for a 303 Savage. Don't know whether one exists. Have read on this forum that the spec calls for a 0.311 in projectile so there is some kind of SAMMI comparison. Just as a Swag, I'd gues a +/- dimension of between .003 to .005 for a Sammi spec length for this cartridge. This one may be a little longer. Yes, on a rimmed case, actual headspace is controlled by the depth of the rim--its thickness--and the condition of both the bolt face and the rim face of the chamber. What these 2 manuals quote as headspace is the distance from the back of the cartridge rim to a datum point of 0.375 dia. located on the shoulder, like with a rimless necked cartridge. Now, I don't know what was done by all manufacturers back in the day, but these Rem rounds I have closely mirror the PPU in length from the back of the head to the datum line. As you so well pointed out, case measurement is a poor way to evaluate case pressures, but until case growth is limited, pressures will certainly vary as will velocity and accuracy. Case capacity will increase to some degree and that too will provide more variance in velocity and accuracy. Makes a fellow wonder why the manu would make it so short. If growth cannot be restricted, then even with out over working the brass, head separations will occur. This PPU is good heavy brass, but it won't stretch but so far.
